Copy 1 


- IITED STATES DEPARTMENT OF AGRICULTURE 

BULLETIN No. 956 


Contribution from the Bureau of Plant Industry 
JVM. A. TAYLOR, Chief 


Washington, D. C. ▼ August 17, 1921 


A STUDY OF THE FACTORS 
AFFECTING TEMPERATURE CHANGES 
IN THE CONTAINER DURING THE 
CANNING OF FRUITS AND 
VEGETABLES 

' ' / ,; By 

C. A. MAGOON and C. W. CULPEPPER 
Office of Horticultural and Pomological Investigations 


CONTENTS 

Page , Page 

1 Single-Period Processing. 17 

2 Intermittent Processing. 45 

6 Summary. 53 

10 Literature Cited. 55 


Basis of the Study. . . . 
Review of the Literature 
Methods and Apparatus 
Preliminary Experiments 




WASHINGTON 

GOVERNMENT PRINTING OFFICE 
1921 




























< 


BUREAU OF PLANT INDUSTRY. 


William A. Taylor, Chief. 

K. F. Kellerman, Associate Chief. 

James E. Jones, Assistant in Charge of Business Operations. 
J. E. Rockwell, Officer in Charge of Publications. 


Office of Horticultural and Pomological Investigations. 

SCIENTIFIC STAFF. 

L. C. Corbett, Horticulturist in Charge. 


Truck-Crop Production Investigations : 

J. H. Beattie. 

C. J. Hunn. 

Irish-Potato Production Investigations : 
William Stuart. 

C. F. Clark. 

W. C. Edmundson. 

P. M. Lombard. 

Truck-Crop Improvement Investigations: 

D. N. Shoemaker. 

Landscape-Gardening and Floriculture In¬ 
vestigations : 

F. L. Mulford. 

W. Van Fleet. 

B. Y. Morrison. 

Bulb-C#lture Investigations: 

David Griffiths. 

Fruit and Vegetable Utilization Investiga¬ 
tions : 

J. S. Caldwell. 

C. A. Magoon. - 
C. W. Culpepper. 

Fruit-Production Investigations : 

H. P. Gould. 

C. F. Kinman. 

George M, Darrow. 


Grdpe-Production Investigations : 

George C. Husmann. 

Charles Dearing. 

F. L. Husmann. 

Elmer Snyder. 

Fruit Breeding and Systematic Investiga 
tions in Pomology : 

W. F. Wight. ’ * 

Magdalene R. Newman. 

Fruit Improvement through Bud Selection 
A. D. Shamel. 

C. S. Pomeroy. > 

R. E. Caryl. 

Nut-Production Investigations : 

C. A. Reed. 

E. R. Lake. 

M. N. Wood. 

Fruit and Vegetable Storage Physiology: 

L. A. Hawkins. 

R. C. Wright. 

J. R. Magness. * 

G. F? Taylor. 

J. F. Fernald. 

H. C. Diehl. 

Nursery-Stock Investigations : 

L. B. Scott. 

G. L. l r erkes. 


Extension Work (in cooperation with States Relations Service) : 

W. R. Beattie. ' 

C. P. Close. 


f*" 


*.uttWiTh 




imm oucQNQfea 

OOeWWtwrs DiViffON 

nHi ri 111111111 iiii r rmii iiwn n i ; 









UNITED STATES DEPARTMENT OF AGRICULTURE 


BULLETIN No. 956 

Contribution from the Bureau of Plant Industry 
WM. A. TAYLOR, Chief 


Washington, D. C. V . August 17, 1921 




A STUDY OF THE FACTORS AFFECTING TEMPER¬ 
ATURE CHANGES IN THE CONTAINER DURING 
THE CANNING OF FRUITS AND VEGETABLES. 1 

By C. A. Magoon and C. W. Culpepper, Office of Horticultural and Pomological 

1 nves tiga t i ons. 


CONTENTS. 

Page. 


Basis of the study- 1 

Review of the literature- 2 

Methods and apparatus_ 6 

The steam retort- 7 

The brass fitting- 8 

Thermocouples- 10 

The water bath- 10 

Preliminary experiments_ 10 

Distilled water_ 10 

Brine- 14 

Sugar solutions- 14 

Starch solutions-1_ 1G 

Single-period processing_ 17 

String beans_ 18 

Peas- 21 

Lima beans_ 24 

Soy beans- 25 

Asparagus- 28 


Page. 


Single-period processing—Contd. 

Sweet corn- 29 

Pumpkin_ 34 

Sweet potatoes- 3G 

Tomatoes_ 39 

Cabbage_ 41 

Factors affecting the change of 
temperature at the center of 

the can_ 42 

Intermittent processing_ 45 

String beans_ 45 

Sweet corn- 47 

Soy beans_ 48 

Sweet potatoes- 50 

Factors influencing the rate of 

change of temperature- 50 

Summary_ 53 

Literature cited- 55 


BASIS OF THE STUDY. 


Successful preservation of foods by canning is due primarily to 
the fact that in the processing, or cooking, the bacteria and other 
microorganisms which cause spoilage are destroyed. Since the 
elimination of these microorganisms is dependent upon the use of 
heat as a sterilizing agents it becomes odF paramount importance to 
know just ivhat temperatures and processing periods will destroy 
them. If uniformly good results are to be expected, a sufficient de¬ 


gree of heat must penetrate.to. all part^ftfethe can or jar, and must 




i The manuscript of this bulletin was submitted for publication on February 27, 1920; 
circumstances of an incidental character interfered with its early issue. 


44900°— 21 


1 














































2 


BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. \ 

be maintained long enough to render all microorganisms harmless. 
Before an accurate judgment as to the proper cooking period can be 
formed it is necessary to know how long a time is required for the 
food at the center of the container to reach the temperature of the 
retort or water bath in which it is being processed. 

In the work here reported upon attention has been centered upon 
these time-temperature relations, and the purpose has been to bring 
to light underlying principles rather than to lay down definite rules 
of procedure, for specific recommendations should be preceded by 
carefully demonstrated facts. 

The diagrams and other data presented are based upon the results 
of more than 600 tests made during the year 1919. All vegetables 
used were grown especially for this work on the experimental farm 
at Arlington, Va., and these, together with such fruits as were used, 
were handled fresh from the fields. In the details of preparation 
of these materials for the tests, no attempt was made to follow any 
particular set of rules hitherto laid down. Bather, an endeavor was 
made to illustrate average conditions, and the results of this experi¬ 
mentation are offered in the hope that they may be of service to 
other workers in this field. 

REVIEW OF THE LITERATURE. 

Attention was first called to the importance of the rate of heat 
penetration into cans of food material during processing by Prescott 
and Underwood (12, 7) 2 in 1898. In a study of the cause of souring 
in canned corn these authors went thoroughly into the bacteriology 
of the problem and isolated and studied the causal organisms found. 
A long series of experiments was undertaken involving more than 
400 tests, in which, by the use of maximum thermometers sealed into 
the cans, the length of time required for the temperature at the center 
of the cans of corn to reach that of the retort was determined. Their 
results showed that, whatever the temperature of processing, the 
center of the cans of corn reached the temperature of the retort in 
approximately the same time. This observation has been made by 
later investigators and accords with the findings reported in this 
work. 

These workers found that in processing corn at 118.8° C. for 1 
hour 55 minutes were required for the center of the 2-pound can to 
reach the temperature of the retort, and they concluded that in 
processing for 1 hour the maximum temperature was maintained for 
only 5 minutes. They failed to take into consideration, however, the 
fact that in substances of heavy consistency, such as corn, the tem¬ 
perature is maintained for a considerable time at or very close to the 
temperature of the retort after removal from it to the air. 


3 Serial numbers in parentheses refer to “ Literature cited ” at the end of this bulletin. 




TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 3 


Iii the various discussions of their work the fact was also pointed 
out that the quantity of liquor added to corn affects the rate at which 
heat penetrates into it. 

Duckwall (9) in 1905 reported similar experiments with peas. He 
also found that, regardless of the temperature of processing, the 
temperature in the can reached that of the retort in the same length 
of time. 

In the same year Belser (1), working upon the spoilage of canned 
foods, also reported upon studies of the time-temperature relations in 
the cans. He worked with peas, beans, mixed vegetables, carrots, 
tomato puree, comfrey, spinach, sauerkraut, cherries, and apple pulp. 
Maximum temperatures during the processing were obtained by the 
use of maximum thermometers, and numerous tables of results were 
given. These show, as might be expected, considerable variations in 
the temperatures reached by the different foods when handled under 
identical conditions. Belser pointed out the great importance of know¬ 
ing the speed at which the heat penetrates to the center of the cans 
and performed several experiments with peas and beans to de¬ 
termine this. The method of preparation of material, the nature of 
the containers, and the details of his work were such as to make 
direct comparisons with American work impossible. 

Haselhoff and Bredemann (10) in their report of investigations 
upon the decomposition of canned foods (1906) referred briefly to an 
apparently unpublished work of Huber in which attention was called 
to the fact that during the processing of certain food products the 
temperature inside the containers often did not reach that of the 
bath or retort in which they were processed. 

Kochs and Weinhausen (11) carried on experiments (1906-7) 
with cabbage, carrots, asparagus, apple sauce, and peas. The 
methods employed correspond closely to those of Belser (1), and 
their results are not comparable with the findings of American 
workers. They pointed out that the rate of heat penetration is de¬ 
pendent upon the firmness of the pack and the proportion of liquid 
present. They worked also with glass and stoneware containers. 

Bitting (2), in 1912, described two methods for the determination 
of the rate of heat penetration in cans of food. One method made 
use of long-stemmed thermometers held with the bulb at the center 
of the cans by means of a special device whereby direct reading of 
the temperature was made possible. The second method made use 
of thermocouples. For the higher temperature chlorid baths were 
used. Bitting pointed out that in substances having plenty of free 
liquid the heat passed in much more rapidly than in substances of 
heavy consistency and with less liquid. The advantages of agita¬ 
tion in shortening the cooking period were emphasized. No ex¬ 
perimental data were given, however. 


4 BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 

Zavalla (14), in 1910, published tables of experiments with cher¬ 
ries, apricots, peaches, and pears. He found the time required for 
the temperature to reach 212° F. at the center of the can processed in 
boiling water to vary with the different fruits and explained his 
findings as due to the difference in their heat conduction. He also 
stated that the concentration of the sirup seemed to exert a definite 
action upon the rapidity with which the heat penetrated to the center 
of the can. His conclusions agree with the results of the present 
work, but the tables given are not sufficiently clear to make direct 
comparisons possible. 

Bitting and Bitting (4), using both thermometers and thermo¬ 
couples, made numerous tests with various fruits and vegetables 
(1917). They worked out experimentally the effect of agitation upon 
the rate of heat penetration, and found that about 12 revolutions 
per minute gave the most satisfactory results in getting the heat to 
the center of the cans without injuring delicate fruits and berries. 
Numerous diagrams were given, showing the results of processing 
various substances with the cans held stationary and also rolled. 
These investigators stated that the minimum time was required to 
bring the temperature of the can to that of the surrounding bath in 
those foods in which the proportion of liquid allowed free convec¬ 
tion and that mashed sweet potatoes required about the maximum. 
Furthermore, they found that in sweet potatoes the temperature at 
the center of the can rarely reaches to within 10 degrees centigrade 
of that of the retort or bath during the ordinary processing. Their 
results are entirely in accord with the conclusions drawn from the 
present work. 

Denton (8) in 1918 reported the results of two tests with carrots 
in which the influence of closeness of pack on heat penetration was 
considered. 

During the same year (1918) Bovie and Bronfenbrenner (5) de¬ 
scribed a thermoelectric apparatus for measuring the rate of heat 
penetration in foods during the canning process. The apparatus 
allowed the determination of the temperature at various parts of the 
can at any time during the processing by means of thermocouples. 
Measurements closer than 1° C. were not attempted, however, and 
the “constant” junction was placed in the autoclave close to the 
test can. Inasmuch as 15 minutes were required to obtain an equi¬ 
librium of pressure and therefore temperature, the “constant” junc¬ 
tion did not become constant until 15 minutes after the can was placed 
in the retort. 

While this apparatus would be satisfactory, perhaps, for sub¬ 
stances like baked beans and sweet potatoes, it would be entirely 
unsuited for the determination of temperature changes in cans of 
such products as string beans and peas, in which the temperature in 


temperature changes in canning fruits and Vegetables. 5 


the can attains or approaches closely that of the retort in less than 
lo minutes. 1 he prime advantage of the apparatus as described by 
these authors is that it allows the determination of the temperatures 
at various parts of the can at any time, thus giving a true idea of the 
rate of heat flow within the material itself. It is obvious, however, 
that the readings obtained under the conditions described are no 
more accurate than may be obtained by the direct reading of the 
mercury thermometer. 

Castle (6) in 1919 called attention to the fact that the depth of 
the water bath about the jars directly affects the rate of change of 
temperature at the center of the containers, the shallower the bath 
the slower the rate of change. She also pointed out that in the inter¬ 
mittent process the first cooking may so compact the material that 
the heat penetrates more slowly in the second and third heatings. 
This was found to be true for leafy vegetables. No differences were 
obtained in the rate of change of temperature in blanched and un¬ 
blanched string beans, and she erroneously concluded that blanching 
does not permit closer packing of this product. 

Thompson (13) in 1919 published a preliminary report upon a 
large amount of valuable work dealing with temperature-time rela¬ 
tions in various fruits and vegetables during processing, in which he 
made use of thermocouples. From these tests he developed mathe¬ 
matical formulas the object of w T hich was to make possible the cal¬ 
culation of the temperature at the center of the can at any time 
during the processing, starting at any initial temperature. 

Such formulas would be of great value if they could be made 
applicable to the handling of all food substances canned. In using 
such formulas, however, it is necessary to assume that all heat trans¬ 
ferred is by conduction or else that any convection is very local. This 
would make the method inapplicable, apparently, for determining 
temperature changes in cans of substances such as string beans and 
peas, in which there is free convection, and would limit its useful¬ 
ness to the canning of substances of heavy consistency, such as corn 
and squash. Inasmuch as the use of these formulas depends upon a 
constant factor, k (which in itself varies with different methods of 
processing, different containers, different kinds of food materials, 
differences in packs, and in some cases differences in varieties, stages 
of maturity, and other factors), it would seem tliat the establishment 
of the necessary constants would be very difficult and would in itself 
necessitate determinations which would give directly the original 
time-temperature facts desired. Furthermore, in certain substances 
the heat is carried inward during the first part of the processing 
period by convection, and in the latter part almost entirely by conduc¬ 
tion. In other cases there is a change in the material in the can 
during processing resulting in the reverse of this, the heat passing 


6 BULLETIN 956, U. S. DEPARTMENT OP AGRICULTURE. 

in at first by conduction and later by convection. Any formulas 
which take into consideration such factors as these must be very com¬ 
plex, indeed, and their application would be difficult and of doubtful 
value. This investigator may be able to overcome some of these 
difficulties in further work. 

METHODS AND APPARATUS. 

As has been pointed out, the earlier work upon the time-tempera¬ 
ture relations in foods during canning made use of maximum ther¬ 
mometers, which were sealed into the material in the cans. While 
the information obtained in this way is valuable so far as it goes, for 
practical purposes and for the carrying out of careful scientific inves¬ 
tigations the use of the maximum thermometer is out of the ques¬ 
tion. In the first place, for one experiment, which may require from 
one to many hours to complete, only one temperature reading can be 
obtained. In an experiment of this sort nothing is known of the 
exact length of time required for the material at the center of the 
can to reach the recorded temperature or of the length of time the 
temperature may have remained at that point. Furthermore, it is 
necessary to carry out many tests in order to record even a partial 
story of the time-temperature relations in a single can of material. 
To make studies of this kind of the most value, it is important to 
know not only what is the highest point reached, but also something 
of the rate of rise in temperature before that point is reached, and 
especially for how long it remains at or above the pasteurizing or 
sterilizing temperature during the processing. 

To overcome the disadvantages of the maximum thermometer, 
thermocouples have been used in more recent investigations. These 
enable the worker to record the entire story of the temperature 
changes in any part of the can if desired, and when properly stand¬ 
ardized they are highly accurate. The principal drawback in their 
use is the complexity of the equipment, which requires considerable 
technical skill to operate properly and the fact that the equipment 
is not available for many who would care to carry on studies in this 
field. Furthermore, thermocouples must be confined primarily to 
laboratory investigations, as they are unsuited to practical routine 
work. 

With these facts in mind an endeavor was made to devise an ap¬ 
paratus which would be inexpensive to install, simple and easy to use, 
and at the same time sufficiently accurate for the determination of 
temperature changes under various conditions of processing. A 
standard method of determining temperature is by the use of the mer¬ 
cury thermometer, and it should be acceptable for this work, pro¬ 
vided it is suitably constructed, properly calibrated, held securely, 


TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 7 

and so placed as to make direct reading possible at all times during 
the processing. It is recognized, of course, that the mercury ther¬ 
mometer is subject to small inaccuracies, but when it is properly 
constructed and standardized these are too small to be of practical 
importance in work of this sort. In the work here reported special 
long-stemmed thermometers were employed which were calibrated 
for 6-inch immersion and graduated to read from —10° to +150° on 
the centigrade scale. When tested these thermometers showed a lag of 
only 15 seconds in passing from 0° to 100°—a lag even less than that 
of the thermocouple used by Thompson (13). 

The use of long-stemmed thermometers for time-temperature 
studies is, of course, not new. Bitting (2) and others have em¬ 
ployed them for experiments made when processing in the water 
and chlorid baths, but their use for tests carried on heretofore in the 
steam retort has not been found feasible. The difficulties have been 
successfully overcome, and the apparatus here described and illus¬ 
trated shows how the temperature at the center of the can may be 
determined at all times, whether the processing is being done in the 
water bath or in the steam retort. 

THE STEAM RETORT. 

The steam retort used in these experiments was constructed from a 
piece of 8-inch water pipe 14 inches long, fitted at one end with a 
blind flange, which serves as the base of the retort, and at the other 
with a removable blind-flange cover. By means of f-inch wrought- 
iron handles and hinged clamp bolts the cover may be placed in 
position and securely clamped down in a very few seconds. Steam 
from a boiler large enough to furnish an ample supply of steam at any 
pressure desired enters by way of a J-inch pipe inserted in the side 
of the retort a few inches above the base, and an exhaust of the same 
size is provided in the bottom. A carefully tested and standardized 
pressure gauge is also attached. 

In the cover a pet cock allows the rapid expulsion of air from the 
retort and also makes possible a continuous flow of steam about the 
test can during the processing. A 1-inch hole at the center of the 
cover is threaded to receive a special brass fitting to Avhich the test 
can is attached. By means of a suitable gasket the joint is made 
steam tight, and with the brass fitting and the can in place the appa¬ 
ratus is ready for the insertion of the thermometer. The thermometer 
is passed through the brass fitting by way of a f-inch hole until the 
bulb reaches the center of the test can, as determined by careful 
measuring beforehand, the funnel-shaped depression in the stem of 
the fitting provided with a suitable gasket, and the cap screwed 
down. The thermometer is thus held securely in position, and a 
steam-tight closure is easily made. 



8 


BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 



It will be seen, therefore, that the test can is firmly attached to the 
cover of the retort by means of the brass fitting, and the can may be 
placed in or removed from the retort in a very few seconds by simply 
putting on or taking off the cover. Owing to the small size of the 
retort, equilibrium at any steam pressure desired may be attained in 

10 to 30 seconds. 
The top of the mer¬ 
cury column is al¬ 
ways in sight, and 
th e temperature a t 
the center of the con¬ 
tainer may be read 
directly at any time. 
Figure 1 shows the 
arrangement of the 
apparatus a n d t h e 
position of the test 
can in the retort. 


The accompanying 
illustrations (fig. 2) 
show in detail the 
structure of the spe¬ 
cial fitting to which 
the can is attached 
for the test. Nu¬ 
merous modifications 
of this are possible, 
to suit all needs. 
The original form is 
shown at A. The 
threaded stem screws 
into the cover of the 
retort until the hex¬ 
agonal shoulder 
presses upon the 
gasket and forms a 
steam-tight joint with the retort cover. The threaded portion below 
the shoulder screws into the hole of the ordinary maximum ther- 
mometer test can in common use. With suitable gaskets air-tight 
joints are made, and the can may be attached or removed as desired. 
Cans of this type were used largely in these investigations. The 
f-inch hole through which the thermometer is passed is reamed out 


Fig. 1.—Apparatus designed for making time-temperature 
tests in canning food products, showing the position of 
the test can in the retort and the arrangement of the 
long-stemmed thermometer used in taking can tempera¬ 
tures. 


THE BRASS FITTING. 


















































































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 9 

% 

at the upper end to form a funnel-shaped depression to allow suit¬ 
able packing, and the cap screws down to hold this tightly about the 
thermometer and make the seal perfect. In practice, this fitting is 
rarely removed from the retort cover, the can being unscrewed from 
beneath and the thermometer removed by unscrewing the cap. 

It will be seen that this apparatus is as well suited for use with 
thermocouples as with thermometers. 

A modification of A is shown at B , differing from it in that the 
fitting may be soldered directly to an ordinary hole-and-cap can. 

For work with glass jars, those of the Mason screw-top type were 
used. These were attached to the fitting A (fig. 2) by removing the 



F IG . 2 ._Details of the special fitting to which the can is attached for the test: A, The 

original form ; B, a modification of A, differing from it in that the fitting may be 
soldered directly to an ordinary hole-and-cap can. 


porcelain in the top, cutting a hole in the metal large enough to te- 
ceive the portion of the fitting below the shoulder, and securing it 
firmly to the fitting by means of a suitable gasket and nut. The jar 
could then be placed in position for the test by simply screwing it 

into the top. 

This apparatus as described may be constructed in any well- 
equipped machine shop at small expense. It is easy to opeiate, xe 
quires no special training for carrying out the tests beyond that pos¬ 
sessed by anyone familiar with canning operations, and is sufficiently 
accurate for all practical needs. Tests made by its use have been 
carefully checked with thermocouples, and the differences observed in 
the results have been too small to be of practical significance. Any- 

44900°—21-2 























































































10 BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 

one, therefore, desiring to undertake time-temperature studies with 
canned foods need not hesitate to make use of this or a similar 
device. 

THERMOCOUPLES. 

The thermocouples used were constructed of copper and constantan 
wires by Dr. R. B. Harvey, of the Office of Plant Physiological 
and Fermentation Investigations, to whom the writers are greatly 
indebted for assistance in installing and standardizing the thermo¬ 
electrical equipment. The constant junction was located in an ice 
bath maintained at 0° C. by means of a thermos bottle filled with an 
ice and water mixture, and the variable junction was placed in the 
center of the container of material under test. The potential was 
measured with a potentiometer of recognized standard and a re¬ 
flection type of galvanometer. 

THE WATER BATH. 

In all time-temperature experiments conducted at 100° C., a water 
bath was used. This consisted of a wooden tank 18 by 18 b}^ 30 
inches lined with galvanized iron and heated by means of a steam 
coil constructed from f-inch pipe. The water was maintained at a 
constant level and kept vigorously boiling throughout the tests. 
Owing to the large volume of water in the bath, there was no cessa¬ 
tion of boiling when the cans and jars were introduced. The tem¬ 
perature was therefore always 100° C., a condition which often does 
not obtain when small kettles or pots are used for the purpose. 

PRELIMINARY EXPERIMENTS. 

To obtain a thorough understanding of the factors influencing tem¬ 
perature changes in the can during the processing period and the 
subsequent cooling it was considered advisable to make some pre¬ 
liminary experiments which would serve as a basis for comparison. 
In these experiments distilled water, brines of various concentrations, 
and solutions of sugar and starch were treated as here described. 
The processing was done in the water bath at 100° C., and in the case 
of distilled water also in the retort at 109°, 116°, and 121° C. 

DISTILLED WATER. 

The first of these experiments was carried out with distilled water, 
using No. 2, No. 3, and No. 10 tin cans and pint and quart glass jars. 
In the case of the tin cans the water was filled to within one-fourth of 
an inch of the top and the device for holding the thermometer 
soldered to the can, making a steam-tight joint. In the glass a steam- 


TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 11 

tight closure could not be made, owing to the inability of the glass 
to stand steam pressure. 

Figure 3 shows the curves representing the temperature changes 
during the processing period and also the cooling in air and in 
water. These represent the averages of six tests for the rise in tem¬ 
perature during processing, but the cooling in water and in air are 
the averages of only three tests. The temperatures of the air and 
of the water in the different cooling tests varied somewhat, so that 
the curves are not absolutely uniform. The initial temperature was 
20° C. The temperature of the water in which the cooling was done 
varied between 15° and 18° C. It is seen that the change in tem¬ 
perature at the center of the can is exceedingly rapid when the can 



Fig. 3.— Time-temperature relations for distilled water when processed in various con¬ 
tainers at 100° C. The curves representing rise in temperature during processing and 
the fall in temperature during cooling in water were plotted from readings made at 
intervals of 30 seconds ; curves representing cooling in air, from readings at intervals 
of 5 to 10 minutes. Rise in temperature : A, In No. 2 tin cans ; B, in No. 3 tin cans ; 
C, in No. 10 tin cans; D, in pint glass jars; E, in quart jars. Cans cooled in 
water: a No. 2 at 17° C; V, No. 3 at 16° C.; o’, No. 10 at 161° C. Cooled in air: 
a, No. 2 cans at 17° to 20° C.; b, No. 3 cans at 16° to 20° C. ; c. No. 10 cans at 16° 
18° C.; d, pint glass jars at 16° to 18° C.; e, quart glass jars at 18° to 20° C. 

is plunged into the water bath at 100° C. In the No. 2 tin can the 
temperature of the bath is approached in about eight minutes. It is 
also noted that the No. 3 can is only slightly slower than the No. 2, 
requiring only two or three minutes longer to attain the temperature 
of the bath. The No. 10 tin can is somewhat slower than the No. 3, 
but even here the temperature at the center approaches that of the 
retort in 15 minutes. The temperature changes in the glass con¬ 
tainers are very much slower than in the tin, requiring about 20 
minutes for the temperature of the center of the pint jar to approach 
that of the water bath, and about 27 minutes for the quart jar to 
reach the same temperature. There is thus a very marked retarda¬ 
tion in the glass. When the tin containers are removed from the 
boiling water bath and placed in water at 1 < ° C. there is a very 




















































12 BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 

sadden drop in the temperature at the center of the can. It falls 
to 30° in a very few minutes, the rate of cooling being only slightly 
slower than the rate of rise in temperature. As will be seen, the 
cooling in air is very slow when contrasted with the cooling in water. 
No cooling tests of glass in water, of course, could be made In 
these tests in the cooling in air the glass cooled considerably faster 
than the tin. The diameter of the No. 2 tin can is less than that 
of the quart glass jar, yet the quart jar cools faster. This may have 
been caused in part by leakage around the cover of the jar, since a 
steam-tight closure was not possible, but it must be due largely to 



Fig. 4. — Time-temperature relations for distilled water when processed in No. 2 tin cans 
at 100°, 109°, 116°, and 121° C. and also when cooled in air and in water. The 
curves representing the rise in temperature during processing and the fall in tempera¬ 
ture during cooling in water were plotted from readings made at intervals of 30- sec¬ 
onds ; curves representing cooling in air, from readings made at intervals of 5 to 10 
minutes. Rise in temperature when processed : A. At 100° C. ; B, at 109° C. ; C, at 
■ 116° C. ; D, at 121° C. Fall when cooled : a'. From 100° C. in water at 17° C. ; b', from 
109° C. in water at 15° C. ; c’, from 116° C. in water at 16° C.; d', from 121° C. in 
water at 16° C. ; a, from 100° C. in air at 17° to 20° C. ; b, from 109° C. in air at 20° 
to 24° C. ; c, from 116° C. in air at 25° C. ; d, from 121° C. in air at 25° C. 

the fact that glass radiates heat faster than tin. The rate of cooling 
in tin is in the order of the diameter of the cans, the No. 10 being 
slowest, the No. 3 next, and the No. 2 fastest. The pint glass jar is 
faster than the quart glass jar or the No. 2 tin can. The temperature 
of the room was not constant, varying between 16° and 20° C. The 
length of time necessary for any container to reach any specific tem¬ 
perature is shown by the curves. 

In figures 4 to T are shown the curves representing the tempera¬ 
ture changes at the center of the various cans of distilled water when 
processed at 100°, 109°, 116°, and 121° C. Since a steam-tight 
closure could not be made in the glass, any temperature above 100° 








































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 13 


fell to 100° C. as rapidly as the temperature of the retort was low¬ 
ered. Ihe temperature at the center of the glass container was thus 



Fig. 5.—Time-temperature relations for distilled water when processed in No. 3 tin cans 
at 100°, 109°, 116°, and 121° C. and also when cooled in water and in air. The 
curves representing the rise in temperature during processing and the fall in tempera¬ 
ture during cooling in water were plotted from readings made at intervals of 30 sec¬ 
onds ; curves representing cooling in air, from readings at intervals of 5 to 10 minutes. 
Rise in temperature when processed : A, At 100° C. ; B, at 109° C.; G, at 116° C.; 
D, at 121° C. Fall when cooled : a', From 100° C. in water at 16° C. ; b\ from 109° C. 
in water at 17° C. ; c', from 116° C. in water at 15° C. ; d', from 121° C. in water at 
17° C. ; a, from 100° C. in air at 16° C'. ; b, from 109° C. in air at 25° C. ; e, from 116° 
C. in air at 25° C. ; d, from 121° C. in air at 25° C. 



T/ME IN M/NurES 

Fig. G.—Time-temperature relations for distilled water when processed in pint glass jars 
at 100°, 109°, 116°, and 121° C. and also when cooled in air. The curves representing 
the rise in temperature during processing were plotted from readings made at intervals 
of 30 seconds; the curve representing cooling in air, from readings at internals of 
5 to 15 minutes. Rise in temperature when processed: A, At 100° C.; B, at 109° C. ; 
C, at 11G° C.; D, at 121° C. a. Fall from 100° C. when cooled in air at 15° to 20° C. 

always at 100° C. when taken from the retort, and the cooling curve 
from this starting point is all that is given. In the case of the tin 



































































































14 


BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE 


cans processed at temperatures above 100° and cooled in air the 
sudden drop in temperature to 100° C. is not noted, the temperature 
falling gradually, and still more slowly as it approaches that of the 
room. These differences in cooling between the tin and glass con¬ 
tainers are to be observed throughout the whole series of experi¬ 
ments. 

One fact of importance shown by the curves is that the length of 
time required for the center of any can to attain the temperature of 
the bath or retort is approximately the same for all the tempera¬ 
tures here used, except that in some tests the boiling-water bath 
required a slightly longer time. 



Fig. 7. —Time-temperature relations for distilled water when processed in quart glass 
jars at 100°, 109°, 116°, and 121° C. and also when cooled in air. The curves repre¬ 
senting the rise in temperature during processing were plotted from readings made 
at intervals of 30 seconds ; the curve representing cooling in air, from readings at 
intervals of 5 to 15 minutes. Rise in temperature when processed : A, At 100° C.; 
B, at 109° C.; C, at 116° C. ; D, at 121° C. a, Fall from 100° C. when cooled in air 
at 18° to 20° C. 

BRINE. 

To determine whether the addition of salt would have any direct 
influence upon the rate of change in temperature in the can, com¬ 
parative tests were made, using distilled water, a 2 per cent brine, 
and a saturated brine. In figure 8 curves for saturated brine and 
distilled water show that the difference between distilled water and 
a saturated brine is insignificant. From the results of these experi¬ 
ments it is concluded that the proportion of salt commonly added to 
canned materials has no direct effect upon the rate of change of 
temperature at the center of the can. 

SUGAR SOLUTIONS. 

In order to get an idea of the possible effect that the addition of 
sugar to canned materials has upon the rate of change of temperature 









































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 15 



in the can, comparative tests were made with 10 per cent, 30 per cent, 
and 60 per cent cane-sugar solutions. This series of tests was made 
with No. 3 tin cans in the boiling-water bath as the 
heating medium, and in circulating water at 17° C. 
as the medium for cooling. In each of the tests 
the cans were filled with the solution to within 
one-fourth of an inch of the top. The device for 
holding the thermometer was soldered in place and 
the thermometer so placed that the bulb was at the 
center of the can. The curves in figure 9 show the 
results obtained. The sugar solutions show no 
marked effect upon the rate of changes of temper¬ 
ature at the center of the can, where the concen¬ 
tration is 10 per cent or less. Even in a 60 per cent 

solution of sugar 
the effect is less 
marked than 
might be ex¬ 
pected. The ef¬ 
fect of the sugar 
solution upon the 
rate of change of 
temperature at 
the center of the 
can is due to the 
greater viscosity, 
which decreases 



T/ME /MM/MUTES 
Fig. 8. — Time-tem¬ 
perature relations 
for distilled water 
and saturated brine 
when processed in 
No. 3 tin cans at 
100° C. These 
curves were plotted 
from temperature 
readings made at 
intervals of 1 min¬ 
ute. A, Distilled 
water; B, satu¬ 
rated brine. 


JO 20 30 *70 GO SO 70 

T/ME /N M/MUTES 

Fig. 9. —Time-temperature relations for 
10 per cent, 30 per cent, and 60 per 
cent cane-sugar sirup when processed 
in No. 3 tin cans at 100° C. and also 
when cooled in water. These curves 
were plotted from temperature read¬ 
ings made at intervals of 1 minute. 
Rise in temperature: A, For 10 pel* 
cent sirup; B, for 30 per cent sirup; 
C, for 60 per cent sirup. Fall in tem¬ 
perature from 100° when cooled: 
a'. For 10 per cent sirup in water at 
12 1° C.; for 30 per cent sirup in 

water at 17° C.; c', for 60 per cent 
sirup in water at 151° C. 


the rate of convection in the sugar 
solutions. The value of the force 
which tends to produce convection 
currents in the solution depends 
upon the steepness of the gradient 
between the temperature at the 
center of the can and the tempera¬ 
ture at the margin of the solution, 
so that the force tending to produce 
convection currents becomes less and 
less as the temperature at the center 
of the can approaches that of the bath. It is known that the vis¬ 
cosity of the sugar solutions decreases as the temperature increases. 
It is this characteristic of sugar solutions that makes the temperature 
shown by the upward curves follow so closely that of distilled water. 




































16 BULLETIN 956. U. S. DEPARTMENT OF AGRICULTURE. 

In the cooling off, however, there is an increase in the viscosity as 
the temperature falls. Also, as the temperature at the center of the 
can falls the temperature gradient between the center and the mar¬ 
gin becomes flatter; hence, the force tending to cause convection be¬ 
comes smaller and smaller, until finally the resistance due to viscosity 
is great enough to stop all convection, and the process becomes one of 
pure conduction, which is very much slower than convection. A 
pronounced flattening of the curve for 60 per cent sirup at about 30° 
C. is significant. The difference in viscosity at high and low tempera¬ 
tures makes the cooling curve much different from the upward curve. 
It appears from other tests that sugar solutions of 1 to 4 per cent 
when added to materials which are canned have very little effect upon 
the temperature change in the can, but concentration as high as 30 
to 60 per cent will have considerable effect. 



Fig. 10.—Time-temperature relations for 1 per cent, 2 per cent, 3 per cent, 4 per cent, 
and 5 per cent starch solutions when processed in No. 3 tin cans at 100° C. Curves 
A to D were plotted from temperature readings made at intervals of 1 minute and 
curve E from readings at intervals of 5 minutes. Rise in temperature : A, For 1 per 
cent solution ; B, for 2 per cent solution ; C, ifor 3 per cent solution ; D, for 4 per cent 
solution ; E, for 5 per cent solution. 

STARCH SOLUTIONS. 

To study further the effect of viscosity upon the rate of change of 
temperature in the can 1, 2, 3, 4, and 5 per cent starch solutions were 
tested in No. 3 tin cans in the boiling-water bath. No cooling tests 
were made. Carefully dried starch was weighed out and enough 
water added to make 1, 2, 3, 4, and 5 per cent solutions, respectively. 
The starch was gelatinized or brought into colloidal solution by 
heating on a steam bath for one hour, with constant stirring, and at the 
end of this time enough water was added to each lot to equal that lost 
by evaporation. Each lot appeared as a homogeneous grayish semi¬ 
transparent solution or paste. The lots were then cooled to 20° C., 
put into cans, and the thermometer-holding device soldered to the 
cans, as described under “Distilled water.” Figure 10 shows the 
curves for these tests. Each curve represents the average of three 
tests. 








































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 17 


It is seen from the curves that the rate of change of temperature 
at the center of the can in 1 per cent and 2 per cent starch solutions 
is not very different from that in distilled water, although there is a 
slight slowing down of the process. Curve C, figure 10, shows that 
the first part of the process of the 3 per cent solution is very rapid, 
but when the center of the can reaches about 92° C. there is a very 
marked slowing down of the rate of rise of temperature. Also in 
curve 71, representing the 4 per cent starch solution, the first part 
of the process is very rapid, but when the temperature at the center 
of the can reaches about 80° C. there is a marked slowing down of its 
rate of rise. It stops almost entirely at about 83° and remains there 
for 10 to 15 minutes; then it begins to rise again, and gradually ap¬ 
proaches the temperature of the bath. In the 5 per cent solution the 
process is slow from the beginning. It is clear that in the 1 per cent 
and 2 per cent solutions and in the first part of the processes of the 
3 per cent and 4 per cent solutions convection is occurring, which 
explains the rapid rise in temperature at the center of the can. In 
the 5 per cent starch solution and in the last part of the process of 
the 3 per cent and 4 per cent solutions convection is not occurring to 
any great extent, and the heat reaches the center of the can by con¬ 
duction only. In the 4 per cent solution the resistance due to the 
viscosity is not great enough to stop convection in the first part of 
the process. The force tending to produce convection becomes less 
and less as the temperature at the center of the solution approaches 
the temperature of the bath. Hence, convection continues in the 
solution until this force becomes so small that it fails to overcome 
the resistance due to viscosity, when the process of convection stops. 
Then the heat is conveyed only by conduction. This is probably what 
happened in the 3 per cent and 4 per cent starch solutions. Further 
change in the starch may have been a factor, as no tests were made 
to determine whether the starch solution had reached its maximum 
viscosity in the preliminary treatment. The curves for cooling in 
water would have been interesting, but unfortunately they were not 
made. 

From these preliminary experiments it is concluded that the 
factors affecting the rate of change of temperature at the center of 
the can are the diameter of the container, the conductivity, thick¬ 
ness and radiative power of its walls, the temperature, conductivity, 
and mobilitv of its contents, and the temperature, conductivity, and 
movement of the medium surrounding it. 


SINGLE-PERIOD PROCESSING. 

With the facts of the preliminary experiments in mind, work was 
done with the various fruits and vegetables commonly canned, for 

44900°—21-3 



18 


BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 


the purpose of determining to what extent these factors are of im¬ 
portance in actual canning practice. No attempt to follow exactly 
the procedure of any specific canning method was made, the object 
being to get at the underlying principles and fundamental factors 
of the time-temperature relations rather than to check up on pre¬ 
vailing methods. 

STRING BEANS. 


In the tests with string beans the Green Pod Stringless variety 
was used. The beans were gathered from the field, brought into the 
laboratory, washed, broken into pieces 1 to 1J inches long, and then 



Fig. 11.— Time-temperature relations for string beans in 2 per cent brine when processed 
in No. 2 tin cans at 100°, 109°, 116°, and 121° C. and also when cooled in air and in 
water. The curves representing the rise in temperature during processing and the fall 
in temperature during cooling in water were plotted from readings made at intervals 
of i minute and 1 minute; those representing cooling in air, from readings at in¬ 
tervals of 5 to 10 minutes. Rise in temperature when processed : A, At 100° C.; 
B, at 109° C. ; C, at 116° C.; D, at 121° C. Fall in temperature when cooled : a'. From 
100° in water at 17° C. ; V, from 109° in water at 17° C. ; c', from 116° in water at 
161° C.; cl', from 121° in water at 16|° C. ; a, from 100° in air at 16° to 20° C. ; 
b, from 109° in air at 19° to 22° C. ; c, from 116° in air at 18° to 22° C.; d, from 
121° in air at 19° to 22° C. 

blanched for five minutes in boiling water. They were then al- 
loived to cool to room temperature and placed in the cans. Two 
per cent brine was added to fill the interspaces. They were then 
processed at 100°, 109°, 116°, and 121° C. in No. 2 and No. 3 tin 
cans and in pint and quart glass jars. Figures 11 to 14 show the 
rise in temperature at the center of the various containers at the 
different processing temperatures. 

It is observed that the rise of temperature is almost as rapid as that 
of distilled water alone. " In 12 to 13 minutes the temperature at the 
center of the No. 2 tin can approaches or attains that of the retort 
or bath, and in 15 to 16 minutes the temperature of the No. 3 can 




















































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 19 


approaches that of the retort or bath. It requires about 30 minutes 
for the pint glass jar and about 35 minutes for the quart jar to reach 



Fig. 12.—Time-temperature relations for string beans in 2 per cent brine when processed 
in No. 3 tin cans at 100°, 109°, 116°, and 121° C. and also when cooled in water and 
in air. The curves representing the rise in temperature during processing and the 
fall in temperature during cooling in water were plotted from readings made at in¬ 
tervals of i minute and 1 minute; those representing fall in temperature during 
cooling in air, from readings at intervals of 5 to 10 minutes. Rise in temperature 
when processed : A, At 100° C.; B, at 109° C. ; C, at 116° C. ; D, at 121° C. Fall in 
temperature when cooled : a', From 100° in water at 17° C. ; V, from 109° in water 
at lOU C.; o', from 116° in water at 16J° C. ; d', from 121° in water at 16|° C. ; 
a, from 100° in air at 25|° C.; b, from 109° in air at 25i° C. ; o, from 116° in air at 
25J ° C. ; d, from 121° in air at 22° to 26° C. 



Fig. 13.—Time-temperature relations for string beans in 2 per cent brine when processed 
in pint glass jars at 100°, 109°, 116°, and 121° C. and also when cooled in air. The 
curves representing the rise-in temperature during processing were plotted from read¬ 
ings made at intervals of 1 minute ; the curve representing the cooling in air, from 
readings at intervals of 1 to 5 minutes. Rise in temperature when processed: 
A, At 100° C. ; B, at 109° C. ; C, at 116° C.; D, at 121° C. a, Fall in temperature 
from 100° when cooled in air at 18° to 22° C. 

the same temperature. It is evident that so far as time-temperature 
relations in the can are concerned there need be little difference in 























































































20 


BULLETIN 056, U. S. DEPARTMENT OF AGRICULTURE. 


the time for the processing of No. 2 and No. 3 tin cans of string beans. 
A. somewhat longer time obviously should be recommended for the 
glass containers. 

The temperature of the retort or bath is approached in practically 
the same time whether the processing temperature is 100°, 109°, 116°, 
or 121° C. Furthermore, the rise in temperature is so prompt that 
the stirring of the material, as in an agitating cooker, would be of 
no advantage in distributing the heat throughout the can. 

It is to be understood that the temperature here measured is that 
of the liquid surrounding the beans. No record was obtained of the 
actual temperature in the beans themselves, but it would take at most 
only a few minutes longer for the heat to reach the center of the beans. 



Fig. 14.—Time-temperature relations for string beans in 2 per cent brine when processed 
in quart glass jars at 100°, 109°, 116°, and 121° C. and also when cooled in air. The 
curves representing the rise in temperature during processing were plotted from read¬ 
ings made at intervals of 1 minute ; the curve representing the cooling in air, from 
reading's at intervals of 1 to 5 minutes. Rise in temperature when processed: 
A, At 100° C.; B, at 109° C.; G, at 116° C. ; D, at 121° C. a, Fall in temperature 
when cooled in air at 18° to 22° C. 

Records of the temperature for the cooling, both in air and in 
water, were obtained. They are very similar to the cooling of dis¬ 
tilled water, but are somewhat slower. Here, again, the very great 
difference in the rate of cooling in air and in water should be empha¬ 
sized. When the containers were cooled in water the temperature 
fell to 30° C. in 10 minutes or less, while it took more than 3 hours 
to fall to the same temperature in air. In the glass, where a steam- 
tight closure could not be made, any temperature above 100° C. fell 
to 100° as rapidly in the container as in the retort, so that the 
temperature of the glass containers was always 100° when removed 
from the retort. 







































temperature changes in canning fruits and vegetables. 21 

The temperature changes obtained in these tests are easily under¬ 
stood when the composition of the beans and the character of the 
pack are held in mind. The string beans contain only a small pro¬ 
portion of starch or other colloidal material which would readily 
go into solution or gelatinize, so the free liquid in the can is thus 
maintained throughout the processing period. This medium allows 
of convection, which rapidly distributes the heat throughout the can. 

The surface tension between the liquid and the insoluble particles 
of material tends to obstruct convection currents, but since the pieces 
of material in this case are comparatively large the effect of the 
force of surface tension is correspondingly small. However, if the 
material is finely divided, as when it is ground in a food chopper, 
the surface tension is increased to such an extent as to cut down very 
greatly the rate of change of temperature. 

Experiments were also made to determine whether the fullness 
of the pack has any effect upon the rate of change of temperature 
in the center of the can. In cans packed extra full and in cans 
lightly packed the differences were found to be so small as to be 
almost negligible. However, when the beans are thoroughly cooked 
the} 7 may be packed in the can so tightly as to make the interspaces 
filled with liquid more or less discontinuous, in which case there is a 
very marked slowing down of the temperature changes at the center 
of the can. 

. PEAS. 

The variety of garden peas used in these tests was the Early 
Alaska. The peas were gathered from the field as used and brought 
into the laboratory and shelled by hand. In some of the tests the 
peas had somewhat passed the prime stage for canning. No attempt 
was made to grade them. They were blanched five minutes in the 
boiling-water bath and cooled to room temperature. The tin cans 
were filled to within one-fourtli of an inch, and the glass jars to 
within half an inch of the top. Then enough 2 per cent brine was 
added to cover the peas. Each kind of container was processed at 
100°, 109°, 116°, and 121° C. They were each cooled in air, and the 
tin cans were also cooled in water. Figure 15 shows the time- 
temperature record of a No. 2 can during the processing period at. 
the various temperatures and also the cooling in air and in water. 
Figures 16, IT, and 18 show the temperature curves for the process¬ 
ing period in the No. 3 tin can and in the pint and quart glass jars, 
respectively. 

It is observed from these curves that the temperature rises very 
rapidly. The No. 2 tin can approaches the temperature of the retort, 
or bath, in about 12 minutes, the No. 3 can in about 15 minutes, the 
pint glass jar in 30 minutes, and the quart jar in about 35 minutes. 


22 BULLETIN 056, U. S. DEPARTMENT OF AGRICULTURE. 

The temperature changes are like those in string beans except that 
the time required to pass through the last degree is in most cases very 
much longer in the peas than in the string beans. Some viscous 
colloidal material cooks out into the free liquid during the processing 
period. The liquid seems to reach such viscosity as to stop convection 
currents at this point. The viscosity does not have to be very great 
in order to stop convection when the difference in temperatures at the 
center of the can and at the margin is one degree or less. 

The difference in cooling in air and water is very marked, as is 
shown by the curve in figure 15. The curves for cooling in water 
show a marked slowing down of the fall of temperature when it 



TIME IN MINUTES 


Fig. 15. —Time-temperature relations for peas in 2 per cent brine when processed in 
No. 2 tin cans at 100°, 109°, 116°, and 121° C. and also when cooled in water and in 
air. The curves representing the rise in temperature during processing and the fall 
in temperature during cooling in water were plotted from readings made at intervals 
of \ minute and 1 minute ; those for cooling in air, from readings at intervals of 
5 to 10 minutes. Rise in temperature when processed: A, At 100° C. ; B, at 109° C.; 
C, at 116° C. ; D, at 121° C. Fall in temperature when cooled : a’, From 100° in water 
at 20° C. ; b', from 109° in water at 17° C. ; c', from 116° in water at 20° C. ; d', from 
121° in water at 19° C.; a, from 100° in air at 2G° to 30° C. ; b, from 109° in air at 
24° to 28° C. ; e, from 116° in air at 24° to 28° C. ; d, from 121° in air at 24° to 28° C. 

reaches about 45° C., in those curves cooling from 121°, 116°, and 
109° C. The one processed at 100° does not follow the same course 
as the other three. During the processing at the higher temperatures 
some soluble colloidal material was cooked out, which formed a solu¬ 
tion so viscous as to stop convection at this point in the cooling. This 
does not occur in every case, and the rather mature condition of the 
samples used in these cases perhaps explains its occurrence here. 

As in string beans, there need be little difference in the length of 
the processing period for No. 2 and No. 3 tin cans, as far as the rate of 
change of temperature at the center is concerned. The time for the 
processing of glass jars, however, should be longer. 














































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 23 

Stirring the material during processing is of no advantage, be¬ 
cause there is no difficulty in getting the heat to the center of the can. 

It must be remembered also that the cooling is much faster in air 
than in the retort. If the cans are left packed in the retort to cool, 
the temperature may remain above 100° C. for 1 hour or longer. 
If the processing has been sufficient, a rapid cooling is of advantage, 



Fig. 16.—Time-temperature relations for 
peas in 2 per cent brine when proc¬ 
essed in No. 3 tin cans at 100°, 109°, 
116°, and 121° C. These curves were 
plotted from temperature readings at 
intervals of 1 minute and 1 minute. 
Rise in temperature when processed : 
A, At 100° C.; B, at 109° C. ; C, at 
116° C. ; D, at 121° C. 



Fig. 17.—Time-temperature re¬ 
lations for peas in 2 per cent 
brine w T hon processed in pint 
glass jars at 100°, 109°, 

116°, and 121° C. These 
curves were plotted from 
temperature readings made 
at intervals of J minute 
and 1 minute. Rise in tem¬ 
perature when processed: 
A, At 100° C. ; B, at: 109° C.; 
C, at 116° C. ; D, at 121° C. 


because the high temperature continues to alter the flavor and quality 
of the product. 

Here, again, the temperature records shown are for the liquid fill¬ 
ing the spaces between the peas, no attempt being made to measure 
the temperature within the peas themselves. This could have been 
done with suitable thermocouples, but it could not take more than a 
very few minutes for the heat to be conducted from the surrounding 















































24 


BULLETIN 956, 1J. S. DEPARTMENT OF AGRICULTURE. 


liquid to the center of the peas. If the peas are fresh and sound the 
organisms to be destroyed would be on the surface and not in the 
center of the peas. 



Fig. 18.—Time-temperature rela¬ 
tions for peas in 2 per cent 
brine when processed in quart 
glass jars at 100°, 109°, 116°, 
and 121° C. These curves were 
plotted from temperature read¬ 
ings made at intervals of \ min¬ 
ute and 1 minute. Rise in tem¬ 
perature when processed : A, At 
100° C.; B, at 109° C.; 0, at 
116° C. : D, at 121° C. 



Fig. 19. — Time-tem¬ 
perature relations 
for Lima beans in 
2 per cent brine 
when processed in 
No. 2 tin cans at 
100°, 109°, 110°, 

and 121° C. These 
curves were plotted 
from temperature 
readings made at 
intervals of h min¬ 
ute and 1 minute. 
Rise in tempera¬ 
ture when proc¬ 
essed : A, At 100° 
C.; B, at 109° C. ; 
C, at 116° C. ; D, 
at 121° C. 



Fig. 20. — Time-tempera¬ 
ture relations for Lima 
beans in 2 per cent 
brine when processed 
in No. 3 tin cans at 
100°, 109°, 116°, and 
121° C. These curves 
were plotted from tem¬ 
perature readings made 
at intervals of b min¬ 
ute and 1 minute. Rise 
in temperature when 
processed: A, At 100° 
C. ; B, at 109° C. ; C, 
at 316° C.; D, at 
121° C. 


LIMA BEANS. 

The variety of Lima beans used in these tests was the Dwarf Garden 
King. The beans were full grown but not dry and were in prime 
condition for table use. They were gathered, brought into the labor¬ 
atory, and shelled by hand. The shelled beans were washed and 
packed tightly into the cans without being blanched, and only enough 
































































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 25 


2 per cent brine added to cover the beans. They were processed in 
the same way as the string beans. .The results are shown in figures 
19 to 22. 

It will be seen from these curves that the rate of temperature 
change in Lima beans does not differ essentially from that in string 
beans. The stirring of the material during processing is unnecessary. 
Cooling tests in water were not made, and the curves for cooling in 



T/A/AT //V M///C/TES 


Fig. 21. — Time-temperature rela¬ 
tions for Lima beans in 2 per 
cent brine when processed in 
pint glass jars at 100°, 109°, 
11G°, and 121° C. These cuiwes 
were plotted from temperature 
readings made at intervals of 
7. minute and 1 minute. Rise in 
temperature when processed: 
A, At 100° C.; B, at 109° C.; 
C, at 110° C. ; D, at 121° C. 



Fig. 22. — Time-temperature rela¬ 
tions for Lima beans in 2 per¬ 
cent brine when processed in 
quart glass jars at 100°, 109°, 
110°, and 121° C. These curves 
were plotted from temperature 
readings made at intervals of 
l minute and 1 minute. Rise in 
temperature when processed: 
A, At 100° C.; B, at 109° C.; 
V, at 116° C.; D, at 121° C. 


air, which were found to be very similar to those for string beans, 
are omitted, as they add nothing of value. 


SOY BEANS. 


The variety of soy beans used in these tests was the Lasy ( ook. 
The beans were gathered when most of the pods were beginning to 
turn yellow. They were brought into the laboratory, spread upon 

44900°—21-4 



























































26 BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 

trays, and steamed for 5 minutes to soften the hulls so that the 
shelling could be done more readily. The shelled beans were filled 
into the cans and enough 3 per cent brine was added to cover the 
beans. The usual tests in No. 2 and No. 3 tin cans and pint and quart 
glass jars at 100°, 109°, and 121° C. were carried out. No cooling 
in water was made. Figures 23 to 26 show the rise in temperature 
at the center of the cans for both tin and glass containers. 



T/ME JNM/NUTES 


Fig. 23.—Time-temperature relations for 
soy beans (Easy Cook) in 3 per cent 
brine when processed in No. 2 tin cans 
at 100°, 109°, and 121° C. These 
curves were plotted from temperature 
readings made at intervals of \ min¬ 
ute and 1 minute. Rise in tempera¬ 
ture when processed : A, At 100° C.; 
B, at 109° C. ; D, at 121° C. 



T/ME /N M/NUTES 

Fig. 24.—Time-temperature relations for 
soy beans (Easy Cook) in 3 per cent 
brine when processed in No. 3 tin cans 
at 100°, 109°, and 121° C. These curves 
were plotted from temperature readings 
made at intervals of \ minute and 1 
minute. Rise in temperature when proc¬ 
essed : A, At 100° C.; B, at 109° C. ; D, 
at 121° C. 


The temperature rises very rapidly during the first part of the 
processing period. When it approaches to within 2 or 3 degrees of 
that of the retort, or bath, the rise in temperature is much slower 
than in the string beans. The soy beans contain a very soluble 
protein which quickly cooks out into the surrounding liquid, the 
viscosity of which soon becomes such as to stop all coiwection cur¬ 
rents. The heat then passes inward by conduction, which is com¬ 
paratively slow. The cooling in air is considerably slower than in 
string beans or peas. No cooling in water was made, but it would 



















































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 27 


very probably have been much slower than for string beans, for the 
cooking out of viscous colloidal substances greatly affects the rate of 
cooling. 



77/!/.£- 7/V M/A/C/TES 


Fig. 25. —Time-temperature rela¬ 
tions for soy beans (Easy Cook) 
in 3 per cent brine when proc¬ 
essed in pint glass jars at 100°, 
109°, 116°, and 121° C. These 
curves were plotted from tem¬ 
perature readings made at in¬ 
tervals of i minute and 1 min¬ 
ute. Rise in temperature when 
processed: A, At 100° C. ; B, at 
109° C.; C, at 116° C.; D, at 
121° C. 



T/ME/N M//VUTES 


Fig. 26.—Time-temperature re¬ 
lations for soy beans (Easy 
Cook) in 3 per cent brine 
when processed in quart 
glass jars at 100°, 116°, and 
121° C. These curves were 
plotted from temperature 
readings made at intervals 
of l minute and 1 minute. 
Rise in temperature when 
processed: A, At 100° C. ; 
C, at 116° C. ; D, at 121° C. 



Fig. 27. —Time-tem¬ 
perature relations 
for asparagus in 2 
per cent brine when 
processed in No. 2 
tin cans at 100°, 
109°, and 116° C. 
These curves were 
plotted from tem¬ 
perature readings 
made at intervals 
of i minute and 1 
minute. Rise in 
temperature when 
processed: A, At 
100° C.; B, at 109° 
C.; C, at 116° C. 


The differences in No. 2 and No. 3 tin cans and pint and quart 
glass jars are the same as noted in string beans, and the same con¬ 
clusions can be drawn as to the length of time for processing. 

Experiments have shown that the readiness with which viscous 
materials cook out in soy beans varies considerably in the different 
varieties and with the different stages of maturity. Of the several 
varieties tested the Easy Cook is the softest and the cooking out is 





























































28 


BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 

greatest. Also the amount of soluble protein material which cooks 
out in processing seems to increase as the beans approach a mature 
or ripe stage. 

ASPARAGUS. 

Rather comprehensive studies were made with asparagus, which 
included experiments to show the effect of differences in the nature of 



TIME /N M/MUTES 

Fig. 28.—Time-tem¬ 
perature relations 
for asparagus in 2 
per cent brine when 
processed in No. 3 
tin cans at 100° and 
109° C. These 
curves were plotted 
from temperature 
readings made at 
intervals of 1 min¬ 
ute and 1 minute. 
Rise in tempera¬ 
ture when proc¬ 
essed : A y at 100° 
C. ; B, at 109° C. 



T/ME /MM/MUTES 

Fig. 29.—Time-temper¬ 
ature relations for 
asparagus in 2 per 
cent brine when proc¬ 
essed in pint glass 
jars at 100° and 109° 
C. These curves 
were plotted from 
temperature readings 
made at intervals of 
l minute and 1 min¬ 
ute. Rise in tem¬ 
perature when proc¬ 
essed : A, At 100° C.; 
B, at 109° C. 



77/Vif /M M/MUTES 

Fig. 30. — Time-tempera¬ 
ture relations for as¬ 
paragus in 2 per cent 
brine when processed in 
quart glass jars at 109° 
C. This curve was 
plotted from tempera¬ 
ture readings made at 
intervals of 1 minute. 


the pack and the comparative effects of water and of brine upon 
the rate of change of temperature. Tests relating to the rate of cool¬ 
ing both in air and in water under these different conditions were 
also made. No perceptible differences were observed when water and 
when brine were used, or when different kinds of packs, viz, whole 
tips or one-half inch pieces, were employed. One set of curves, there- 












































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 29 


fore, illustrates what was obtained in each case. Figures 27 to 30 
show the curves for asparagus prepared by washing, cutting the stalks 
into one-half inch pieces, and packing into the containers, after which 
2 per cent brine was added to fill the interspaces. 

It will be noted that the curves for asparagus do not differ essen¬ 
tially from those of string beans. The cooling in air and in water also 
gave curves entirely similar to those of string beans. 

Asparagus does not contain any large amount of soluble collodial 
materials to cook out and change the viscosity of the surrounding 
liquid to any great extent, although it easily collapses when it is 
cooked too long or at too high a temperature. 



Fig. 31.—Time-temperature relations for sweet corn (Maine style) when processed in 
No. 2 tin cans at 100°, 109°, 110°, and 121° C. and also when cooled in air and in 
water. The curves representing, the rise in temperature during processing and the 
fall in temperature during cooling in water were plotted from readings made' at inter¬ 
vals of 5 minutes ; those for cooling in air, at intervals of 5 to 10 minutes. Rise in 
temperature when processed : A, At 100° C. ; B, at 109° C.; C, at 116° C. ; D, at 121° C. 
Fall in temperature when cooled : a'. From 100° in water at 15° C. ; V, from 109° in 
water at 22° C. ; c', from 116° in water at 21° C.; , from 121° in water at 18° C.; 

a, from 100° in air at 22° to 26° C.; 6, from 109° in air at 25° to 28° C. ; c, from 116° 
in air at 25° to 28° C. ; d, from 121° in air at 25° to 28° C. 


SWEET CORN. 

The variety of corn used in these tests was Stowell’s Evergreen. 
The corn was picked the same day that the test was made, and only 
ears that were in prime condition for canning were used. It was 
husked, the silks removed with a coarse brush, washed, and then cut 
off the cob “ Maine style,” i. e., about one-third of the grain was cut 
away with a sharp knife and then the rest scraped from the cob. A 
liquor of 2 per cent salt and 6 per cent sugar was prepared and added 
to the corn in the can to give the proportion of 5 parts of corn to 1 
part of liquor. It was then processed in the various containers at 
the different temperatures, as in the previous experiments. Figures 
31 to 34 show the results of these tests. 

































































30 


BULLETIN 056, U. S. DEPARTMENT OF AGRICULTURE. 


These curves indicate that the heat penetrates into the cans very 
slowly. It requires two hours to approach the temperature of the 
retort in a No. 2 tin can and nearly three hours to reach the same 
temperature in the No. 3 can. In one and one-half hours the pint 
glass jar approaches the temperature of the retort and in about two 
hours the same temperature is reached in the quart glass jar. It is 
very evident, therefore, that little convection is taking place. Con¬ 
vection is prevented in part by the finely divided condition of the 
corn, and further by the viscous condition of the liquor, which 
results from gelatinization of the starch. The differences in the rate 
of change of temperature between the No. 2 and No. 3 tin cans are 
very great, indicating the necessity of different processing periods. 



Fig. 32.—Time-temperature relations for sweet corn (Maine style) when processed in 
No. 3 tin cans at 100°, 109°, 110°, and 121° C. These curves were plotted from read¬ 
ings made at intervals of 5 minutes. Rise in temperature when processed: A, At 
100° C. ; B, at 109° C. ; C, at 116° C. ; D, at 121° C. 

The difficulty of getting the heat to the center of a No. 3 can and the 
quart glass jars is so great that it is advantageous to can sweet corn 
in No. 2 tin cans or in pint glass jars. It is to be noted that the 
low conductivity of the glass ceases to be a factor here, and the rate 
of change of temperature follows more nearly the order of the 
diameters of the containers, i. e., the pint glass jar is fastest, then the 
No. 2 tin can, the quart jar next, and the No. 3 can slowest. This 
order is quite different from that of string beans. 

Stirring the material would very greatly aid in getting the heat 
to the center of the can for corn prepared in the Maine style, and for 
this reason agitating cookers might be especially advantageous for 
handling corn packed in this manner. 



































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 31 


Figure 31 shows the record of cooling for a No. 2 tin can in 
air and in water. The cooling in air is considerably slower than that 
of string beans, but the 
differ ences are not so 
great as might be ex¬ 
pected from the differ¬ 
ences in the rise in tem¬ 
perature. The cooling in 
water, although very- 
much faster than the cool¬ 
ing in air, is still very 
slow in comparison to the 
cooling of string beans in 
water. It requires about 
1 hour and 20 minutes for 
the corn to fall to 30° C. 
in these tests, whereas 
string beans required only 
10 to 15 minutes. These 
differences in the rate of 
cooling in air and in 
water. It required about 
tremely important when it 
is remembered that high 
temperatures seriously affect the appearance and flavor of the corn. 

Attention is again called to the fact that when the steam is cut 

off at the end of the 
processing period the 
temperature of the 
jars falls from any 
temperature above 
100° to 100° C. as 
rapidly as the tem¬ 
perature in the re¬ 
tort. This is impor¬ 
tant, especially in 
substances like corn, 
as sterilizing tem¬ 
peratures are main¬ 
tained for much 
shorter periods than 
in tightly sealed 
cans. 

VARIETAL DIFFERENCES. 

Other tests were 
made to determine 



Fig. 34.—Time-temperature relations for sweet corn 
(Maine style) when processed in quart glass jars at 
100°, 109°, 116°, and 121° C. These curves were 
plotted from temperature readings made at intervals 
of 5 minutes. Rise in temperature when processed: 
A, At 100° C. (t he proportion of corn to liquor in this 
case, unfortunately, was less than in those processed 
at the higher temperatures) ; B, at 109° C. ; C, at 
116° C. ; D, at 121° C. 



Fig. 33. —Thne-temperature relations for sweet corn 
(Maine style) when processed in pint glass jars 
at 100°, 109°, 11G°, and 121° C. These curves 
were plotted from temperature readings made at 
intervals of 5 minutes. Rise in temperature when 
processed : A, At 100° C. (the proportion of corn 
to liquor in this case, unfortunately, was less than 
in those processed at the higher temperatures) ; 
B, at 109° C. ; C, at 116° C.; D, at 121° C. 











































































32 


BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 

whether varietal differences in corn had any effect upon the rate of 
change of temperature at the center of the can. Comparison was 
made of White Dent field corn. Golden Bantam, Country Gentleman, 
Stowell’s Evergreen, and Crosby’s Early. No differences of impor¬ 
tance Avere obserA T ed when these varieties were prepared in Maine 
style. Tests with corn at different stages of maturity Avere not made, 
although this would have been of interest, since the starch content is 
known to increase during the approach to maturity. 

It is not probable, however, that the differences in maturity gen¬ 
erally permissible in canning practice Avould haA^e any effect. 

MAINE STYLE AND MARYLAND STYLE COMPARED. 

Having found that differences in the fineness of division of particles 
affects the rate of change of temperature, tests were made for the 

purpose of compar¬ 
ing the Maine style 
and Maryland style 
of packing. Sto- 
Avell’s Evergreen Avas 
the variety used for 
these tests. In the 
Maine stvle the corn 
Avas prepared as al- 
readv described. In 
the Maryland style 
t h e corn was pre¬ 
pared by cutting the 
grains from the cob 
as nearly whole as 
possible, without 
scraping. No. 2 tin 
cans were used, and the same proportion of corn and liquor (by 
weight) was used in each case. The proportion of corn to liquor 
was 2.1 to 1. The results when processed in the water bath at 100° C. 
are slioAvn in figure 35. 

A higher rate of change of temperature is noted in the Maryland 
style than in the Maine style, as the result of the greater freedom of 
movement of the liquor filling the interspaces. There Avas some con- 
A^ection in each case, but the results probably Avould have been entirely 
different if the proportion of water to corn had been other than that 
used in these tests. Such marked differences as are shown in these ex¬ 
periments should be borne in mind Avlien processing periods are under 
consideration. 

EFFECT OF DIFFERENT PROPORTIONS OF LIQUOR. 

Tests Avere made to determine the effect of different proportions of 
corn to liquor upon the rate at Avhich heat passes into the can. The 



Fig. 35.—Time-temperature relations for sweet corn 
(Maine style and Maryland style) processed at 100° C. 
in No. 2 tin cans. These curves were plotted from 
temperature readings made at intervals of 5 minutes. 
Proportion of corn to liquor : A, Maryland style, 2.1 : 1 ; 
B, Maine style, 2.1 : 1 ; C, Maine style, 3.1 : 1. 




























TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 33 

variety used in these tests was Stotvell's Evergreen. The corn was 
prepared in the Maine style, placed in No. 2 tin cans, and processed at 
100° C. The proportions of corn to liquor used in these tests were 
1 to 0,1 to 1, 2 to 1, 3 to 1,4 to 1, and 5 to 1. The results are shown in 
figure 36. 

In the can having the proportion of 1 to 1 there is a very rapid 
rise in temperature, due largely to convection which occurs in the 
liquid. The last two or three degrees go very slowly, liow 7 ever, owing 
to gelatinization of the starch, which increases the viscosity to such 
an extent as to counteract the force, small at this point, tending to 
cause convection. In the can having the proportion of 2 to 1 there is 
a marked falling off in the rate throughout the curve. There is con¬ 
siderable convection here, but it is less pronounced and stops sooner 
than in cans having the proportion of 1 to 1. Likewise in the 4 to 1 
and in the 5 to 1 there is 
a further falling off of 
the rate of temperature 
rise. In these cans con¬ 
vection is further re¬ 
duced and probably plays 
only a small part in the 
5 to 1 cans. It is noted 
that the temperature at 
the center of the can ap¬ 
proaches that of the bath 
more quickly in the case 
of the corn alone than in 
the proportion of 5 to 1, 

4 to 1, or 3 to 1. This 
apparent inconsistency is 
easilv understood when 
one realizes what is taking place in the four cans. In the can . 
having the proportion of 5 to 1 there is little, if any, convection, 
for the going into solution of the starch forms a mass having so 
small an amount of free liquid that very little or no convection takes 
place. In the proportion of 4 to 1 some convection occurs, but it is 
checked early in the process by the same cause. The same in general 
is true in the case of 3 to 1, but more time is required to arrest the 
convection currents. In the corn alone there is no free liquor in 
which convection can take place, but there is a saturated air more or 
less continuous from the center to the outside of the material, which 
certainly allows some convection, thus accounting for the more rapid 
rise in the corn alone than in the 4 to 1 and the 5 to 1 cans. If the 
material in the can of corn alone had been packed down tightly the 
curve would have been considerably different. 



Z/Af£ W M/A/UZES 


Fig. 3G. —Time-temperature relations for sweet corn 
(Maine style) with different proportions of liquor 
processed in No. 2 tin cans at 100° C. These 
curves were plotted from temperature readings 
made at intervals of 5 minutes. Proportion of 
corn to liquor : A, 1: 1; B, 2 : 1 ; G, 3 : 1 ; D, 4 : 1; 
E, 5:1; F, corn alone. 




























34 


BULLETIN I>56, U. S. DEPARTMENT OF AGRICULTURE. 


PUMPKIN. 


The Connecticut Pie pumpkin was used in the tests. The pump¬ 
kins were washed, split into halves, and the seeds removed. They 

were then cut into 
strips, the outer 
rind removed, and 
the pieces steamed 
for 30 minutes. 
After cooling, the 
pieces were ground 
in a food chopper 
in order to get a uni- 
f o r m p u 1 p. This 
material, now in the 
form of pie stock, 
was packed in the 
cans. Figures 37 to 
40 show the results 
of these tests. 

As might be ex¬ 
pected, the rate of 
rise in temperature 
is i r erv slow, there 
b e i n g insufficient 
free liquid to make possible any great amount of convection. The 
time-temperature curves for pumpkin are very similar to those for 



T/ME IN MINUTES 

Fig. 37. —Time-temperature relations ifor pumpkin proc¬ 
essed in No. 2 tin cans at 100°, 109°, 116°, and 121° C. 
These curves were plotted from temperature readings 
made at intervals of 5 minutes. Rise in temperature 
when processed : A, At 100° C.; B, at 109° C. ; C, at 
116° C. ; D, at 121° C. 



Fig. 38.—Time-temperature relations for pumpkin processed in No. 3 tin cans at 100°, 
109°, 116°, and 121° C. These curves were plotted from temperature readings made 
at intervals of 5 minutes. Rise in temperature when processed: A, At 100° C. ; B, at 
109° C.; G, at 116° C. ; D, at 121° C. 


sweet corn. Cooling tests in water were not made, and the curves for 
the cooling in air are omitted, as they add nothing of value. 































































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 35 

The differences in the containers are the same as in the case of 
the sweet corn. The temperature rises fastest in the pint glass jar, 
the No. 2 tin can next, 
then the quart glass jar, 
and most slowly in the No. 

3 tin can. The retarding 
influence of the glass ceases 
to be a factor here. This 
is because the conductivity 
of either the tin or the 
glass is greater than the 
conductivity of the ma¬ 
terial. Hence, the rate of 
temperature change at the 
center of the can follows 
the order of the diameters 
of the containers. This is 
true for sweet corn and 
pumpkin and also for sweet 
potatoes, as will be seen 
later. Sometimes pumpkin 
is concentrated before 
being canned. Evap¬ 
orating the pulp to half 
its original volume would probably have only a small effect upon the 
rate of change of temperature. There is very little convection in the 

material as thus pre¬ 
pared, so it is al¬ 
most certain that 
evaporation to one- 
half would make it 
only slightly slower. 

Some experiments 
were made to de¬ 
termine what effect 
cooking before fill¬ 
ing into th.e can 
would have upon 
the temperature 
changes in the can. 
Cans were filled 
with raw material 
ground in the food 
chopper and wit h 
material which had 
been steamed 30 



Fig. 40.—Time-temperature relations for pumpkin when 
processed in quart glass jars at 100°, 109°, 116°, and 
121° C. These curves were plotted from temperature 
readings made at intervals of 5 minutes. Rise in tem¬ 
perature when processed: A, At 100° C.; B, at 109° C.; 
C. at 116° C. ; D, at 121° C. . 



Fig. 39.—Time-temperature relations for pumpkin 
when processed in pint glass jars at 100°, 109°, 
116°, and 121° C. These curves were plotted 
from temperature readings made at intervals of 
5 minutes. Rise in temperature when processed : 
A, At 100° C. ; B, at 109° C. ; C, at 116° C.; 
B y at 121° C. 




































































36 BULLETIN £>56, U. S. DEPARTMENT OF AGRICULTURE. 

minutes ana then ground in the food chopper and given identical 
processing temperatures. No appreciable differences were observed. 

Tests were also made with summer squash prepared as described 
for the pumpkin. The time-temperature curves are practically the 
same. These curves have been omitted to save space. 

SWEET POTATOES. 

The variety of sweet potato used in these tests was the Nancy Hall. 
The potatoes were washed and steamed 30 minutes. After peeling 
they were allowed to cool and were then ground in a food chopper 
in order to get a uniform mash, commonly known as “pie stock.” 



Fig. 41.—Time-temperature relations for sweet potatoes when processed in No. 2 tin 
cans at 100°, 109°, 116°, and 121° C. and also when cooled in air and in water. These 
curves were plotted from temperature readings made at intervals of 5 minutes. Rise 
in temperature when processed : A, At 100° C. ; B, at 109° C.; C, at 116° C.; D, at 
121° C. Fall in temperature when cooled : a From 100° in water at 15° C. ; V, from 
109° in water at 15° C.; c', from 116° in water at 15° C.; d’, from 121° in water at 
15° C.; a, from 100° in air at 20° to 24° C.; b, from 109° in air at 20° to 24° C.; c, from 
116° in air at 25° to 25J° C. ; d, from 121° in air at 25° to 25^° C. 

Tests in glass were incomplete; hence time-temperature curves for 
these have been omitted. Figures 41 and 42 show the rise in tem¬ 
perature and also the cooling in air and in water for both No. 2 and 
No. 3 tin cans. 

It will be seen that the temperature changes at the center of the 
can are very slow, in most cases slightly slower than in pumpkin 
or sweet corn. The necessity of a considerably longer processing 
period for the No. 3 tin can than for the No. 2 is again emphasized. 
Owing to the firmness of the pack and the absence of free liquid, 
convection currents play no part in the temperature changes here. 
Therefore, rotating the can in order to stir the material, as in an 



















































TEMPERATURE 


CHANGES IN CANNING FRUITS AND VEGETABLES. 


37 


agitating cooker, would have much less effect with sweet potatoes 
than the same treatment with sweet corn. 


COMPARISON OF RAW AND COOKED MATERIAL. 

Tests with sweet potatoes ground in a food chopper and packed 
into the can raw and with material cooked 30 minutes, ground, and 
packed into the can were made, the processing being conducted under 
identical conditions. There were no appreciable differences in the 
rate of change of temperature in the can. It is apparent that the 
gelatinization of the starch has little effect if the nature of the ma¬ 
terial at the outset is such that convection is prevented. 



Fig. 42.—Time-temperature relations for sweet potatoes when processed in No. 3 tin 
cans at 100°, 109°, 116°, and 121° C. and also when cooled in air and in water. These 
curves w T ere plotted from temperature readings made at intervals of 5 minutes. Rise 
in temperature when processed : A, At 100° C. ; B, at 109° C.; C, at 116° C.; D, at 
121° C. Fall in temperature when cooled : a'. From 100° in water at 15° C. ; from 
109° in water at 15° C. ; tfron^ 116° in- water at 19° C.; d' , from 121° in water at 
19° C. ; a, from 100° in air at 18° to 22° C. ; b, from 109° in air at 20° to 24° C. ; o, from 
116° in air at 25° to 251° C. ; d, from 121° in air at 25° to 25i° C. 

PROCESSING FOR DIFFERENT LENGTHS OF TIME AT 116° C. 

Figure 43 shows the result of a series of tests of Xo. 3 tin cans 
processed at 116° C. for 50, 60, 70, 80, 90, and 100 minutes and then 
put immediately into the air. The temperature of the air varied 
somewhat, so that the results are not exactly uniform. Two facts are 
brought out by this series of tests: (1) The temperature continues 
to rise for 20 to 30 minutes after the cans are put into the air; (2) 
at this processing temperature 90 to 100 minutes are required to carry 
the temperature at the center of the can to 100° C., or above. This 
is simply a “ cut and try ” method of finding the length of time neces¬ 
sary to sterilize any particular pack of canned material. If the cans 
had been left in the retort or put into water, the results would have 















































38 


BULLETIN 1)56, U. S. DEPARTMENT OE AGRICULTURE. 


been entirely different. If the initial temperature of the material 
when put into the can had been higher, the maximum temperature 
attained would also have been higher. 



Fig. 43.—Time-temperature relations for sweet potatoes when processed in No. 3 tin cans 
at 110° C. for 50, 60, 70, 80, 90, and 100 minutes and then removed to the air at 
20° to 25° C. These curves were plotted from readings made at intervals of 5 minutes. 
Temperature curve when processed and placed in air : A, For 50 minutes ; B, for 60 
minutes ; C, for 70 minutes ; D, for 80 minutes ; E, for 90 minutes ; F, for 100 minutes. 


It is obvious that the initial temperature in the can should be 
uniform, in all cans of the pack where the same processing is to be 
given, and the initial temperature should also be as high as pos- 



Fig. 44.—Time-temperature relations for sweet potatoes when processed in No. 3 tin cans 
for 1 hour at 100°, 109°, 116°, and 121° C. and then removed .to the air at 20° to 25° C. 
These curves were plotted from readings made at intervals of 5 minutes. Temperature 
curves when processed for one hour: A, At 100° C.; B, at 109° C • C at 116° C * 
D, at 121° C. ’ ' . ’ * 

sible, in order that the processing period may be shortened. Start¬ 
ing at a temperature higher than that of the room is to be recom¬ 
mended. 

PROCESSING FOR ONE HOUR AT DIFFERENT TEMPERATURES. 

Figure 44 shows curves for No. 3 tin cans of sweet potatoes proc¬ 
essed at 100°, 109°, 116°, and 121° C. for 1 hour in the retort, or 































































































TEMPERATURE CHANGES IN CANNING ERUITS AND VEGETABLES. 39 


bath, and then removed immediately to the air. The temperature 
at the center of the can continues to rise for 20 to 30 minutes after 
it is put in the air. When the can is processed for 1 hour at 121° 
the temperature just approaches 100° C. If the can had been left 
in the retort instead of being put in the air, the temperature would 
have gone higher. This shows again the importance and necessity 
of knowing what temperatures are reached during the processing 
period. Such factors as these are often overlooked, when they are 
of very great importance. If the initial temperature had been dif¬ 
ferent, it would have affected the maximum temperature attained. 
In processing sweet potatoes in No. 3 tin cans it is of very great 
importance to have the initial temperature as high as practicable. 

TOMATOES. 

The tomatoes used in these tests were of a special disease-resistant 
variety being studied at the Arlington Experimental Farm. They 
were fully ripened 
and of medium size. 

After scalding for 
two minutes in boil¬ 
ing water they were 
plunged into cold 
water to be peeled. 

After peeling they 
were packed into 
the cans as n early 
whole as possible. 

No water or other 
liquid was added. 

Tests in both tin 
and glass containers 
were made, as usual. 

Cooling tests in air 
only were m a cl e. 

The temperature of 
the air varied considerably, so that the cooling in the various tests 
is not strictly comparable, and the curves were therefore omitted 
from the charts. The results showed, however, that the cooling 
would be somewhat slower than for string beans. Figures 45 to 48 
show the results of these tests. Individual curves represent tests of 
a single can. Duplicate cans varied considerably, owing perhaps to 
inability to pack them exactly alike. The curves illustrate average 
results quite well, however. 

The rate of change in temperature is faster than in pumpkin or 
sweet corn, but very much slower than in string beans. At 100° C. 

7 «y ’ 



Fig. 45. —Time-temperature relations for tomatoes when 
processed in Xo. 2 tin cans at 100°, 109°, 116°, and 
121° C. These curves were plotted from readings made 
at intervals of 5 minutes. Rise in temperature when 
processed : A, At 100° C.; B, at 109° C.; G, at 116° C.; 
D, at 121° C. 

































40 


BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 


it requires about 1 hour and 30 minutes to approach the temperature 
of the bath. At 109° C. it requires about 1 hour and 20 minutes. 



Fig. 46.—Time-temperature relations for tomatoes when processed in No. 3 tin cans at 
100°, 109°, 116°, and 121° C. These curves were plotted from readings made at 
intervals of 5 minutes. Rise in temperature when processed : A, At 100° C.; B, at 
109° C.; C, at 116° C.; D, at 121° C. 


at 116° about 1 hour and 10 minutes, 
to reach the processing temperature 



Fig. 47.—Time-temperature relations for to¬ 
matoes when processed in pint glass jars 
at 100°, 109°, 116°, and 121° C. These curves 
were plotted from readings made at intervals 
of o minutes. Rise in temperature when 
processed: A, At 100° C.; B, at 109° C.; 
C, at 116° C. ; D, at 121° C. 


and at 121° it requires 1 hour 
in a No. 2 tin can. Similar 
results are noticed in the No. 
3 tin cans and in the pint and 
quart glass jars. A shorter 
time is required to reach the 
temperature of the retort at 
121° than at any lower proc¬ 
essing temperature. With 
many vegetables and fruits 
there is a slowing down in 
the rate of rise as the tem¬ 
perature goes higher, owing 
to the going into solution of 
starch, protein, or other ma¬ 
terial, which changes the vis¬ 
cosity of the material. This 
change in viscosity interferes 
with convection, and so the 
process is slowed down. A 
change of exactly the oppo¬ 
site character is taking place 
in the tomato. The tomato 
fruit is very succulent, and 
its tissues are easily broken 
down at high tempera- 
















































































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 41 


tures. There is very little starch, pectin, or other readily soluble 
colloidal material in the tomato. It consists of organic acids 
and sugars, with insoluble cell tissues and a large amount of water. 
As a high temperature is reached the tissues begin to collapse, leav¬ 
ing a free liquid in which convection can take place. This explains 
the greater rate of temperature change at the higher processing 
temperature than at the lower. 

Such high temperatures as 109°, 116°, and 121° C. are not neces¬ 
sary for the sterilization of the tomato, and the results are only of 
theoretical interest. Where it is of importance to keep the tissues 
of the material intact high temperatures should be avoided. It is 
of interest and impor¬ 
tance to know just what 
temperatures are reached 
when the various cans 
are processed for 10, 15, 

20, 25, 30, and 40 minutes 
each, but this work has 
ndt yet been completed. 

The irregularities 
noted in the curves for 
tomatoes in the glass con¬ 
tainers are due to the 
tendency of the material 
to rise to the top when it 
collapses under the high 
temperatures. The solid 
portion as it rises may 
for a time surround the 
thermometer, preventing 
convection, but later rises still higher, leaving the bulb of the ther¬ 
mometer in a liquid that is more or less free. 

CABBAGE. 



Fig. 48. —Time-temperature relations for tomatoes 
when processed in quart glass jars at 100°, 109°, 
110°, and 121° C. These curves were plotted from 
readings made at intervals of 5 minutes. Rise in 
temperature when processed : A, At 100° C.; B, at 
109° C.; C, at 116° C. ; D, at 121° C. 


All cabbage plants used in these tests had firm heads, and the 
outer leaves were discarded. The heads were sliced somewhat 
coarser than for sauerkraut, with a rotary slicing machine. The 
sliced material was then blanched in flowing steam for 10 minutes, 
after which it was packed into the cans and enough water added to 
fill the interspaces. The results are shown in figures 49 and 50. 
The wide variation is due to inability to pack the cans exactly alike. 
In general when packed in this way the rate of change of tempera¬ 
ture at the center of the can is very much slower than that of string 
beans, but considerably faster than that of sweet corn. Cabbage is the 





























42 


BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 


only one of the leafy vegetables that has been included in these tests. 
The rate of change of temperature at the center of the can in the 
leafy vegetables depends upon the nature of the packing. If the 
cabbage in this case had been cut coarser and a little more water had 
been added, the rate of change would have been very much faster. 
On the other hand, if the material had been sliced finer and packed 
into the can a little closer, the rate would have been slower. Any 
alteration of the packing which would affect convection would affect 
the rate of change of temperature at the center of the can. In the 
leafy vegetables almost any results may be obtained between that of 

string beans and 
sweet corn. As 
usually packed, how¬ 
ever, the changes are 
perhaps quite slow. 
This, again, shows 
how important it is 
to know how rapidly 
the temperature i n 
the can approaches 

that of the retort. 

% 

FACTORS AFFECTING 
THE CHANGE OF TEM¬ 
PERATURE AT THE 
CENTER OF THE CAN. 

In considering the 
factors affecting the 
rate of change of 
temperature in the 
can the laws of heat 
transmission, espe¬ 
cially the rapidity of 
convection and the slowness of conduction and radiation, should 
be held in mind. From all the preceding experiments the follow¬ 
ing facts seem clear. 

The first important factor is the size and nature of the container. 
If the rate at which the material in the can transmits heat is slower 
than the conductivity of the walls of the container, then the nature 
of the container ceases to be an important factor and the diameter 
of the container is the chief factor. If the rate at which the material 
carries heat to the center of the can is faster than the conductivity 
of the walls of the container, then the nature of the container is 
important and variation in its conductivity affects the temperature 
changes at the center of the can. Thus in string beans there is a 
difference in the rate of change of temperature in the tin and glass 



Fig. 49.—Time-temperature relations for cabbage when 
processed in No. 2 tin cans at 100°, 109°, 116°, and 
121° C. These curves were plotted from readings made 
at intervals of 5 minutes. Rise in temperature when 
processed : A, At 100° C.; B, at 109° C.; C, at 116° C.; 
D, at 121° C. 































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 43 


due to the differences in the conductivity of the walls of the con¬ 
tainer, but in sweet corn the rate of change of temperature follows 
more nearly the order of the diameter of the container, the glass not 
being an important factor. The diameter of the container is of very 
much less importance in material where there is a free liquid in 
which convection carries the heat rapidly to the center of the can. 
Thus the difference in the rate of change of temperature in No. 2 
and in No. 3 tin cans is so small in string beans that only slight differ¬ 
ences in the processing periods are necessary. 

Variations in the 1 composition of the material are of importance 
when such variations affect convection. If the material is of such 
a nature that no convection occurs, its composition may vary widely 
without greatly affecting the temperature changes in the can. Thus 



T/A/E //V Af/AJUTES 


Fig. 50. —Time-temperature relations for cabbage when processed in No. 3 tin cans at 
100°, 100°, 116°, and 121° C. These curves were plotted from readings made at inter¬ 
vals of 5 minutes. Rise in temperature when processed : A, At 100° C. ; B, at 109° C.; 
C, at 116° C.; D, at 121° C. 

sweet potatoes and pumpkin, though of very different chemical 
composition, have similar time-temperature curves. The going into 
solution of starch may change the viscosity of the material and 
hence affect the temperature changes in the can, but changes in the 
physical nature of the starch have very little effect if the character 
of the pack at the outset is such that no convection can occur. In 
some cases there may be the cooking out of soluble proteins, pec¬ 
tins, or other viscous materials which would interfere with con¬ 
vection. Variations of materials in this respect must be considered 
in processing. Usually in processing where there is a breaking- 
up of the material the rate of change of temperature becomes slower, 
but in the tomato the opposite is true, because the tomato contains 
little starch, pectin, or other mucilaginous material. The liquid be¬ 
comes free, thus allowing convection. 









































44 


BULLETIN 966, IJ. S. DEPARTMENT OF AGRICULTURE. 

The nature of the pack is one of the most important factors af¬ 
fecting the rate of change of temperature. In any material of 
any composition which is so packed that there is a free liquid filling 
in the interspaces between the pieces of material in the pack there 
is a very rapid change in temperature during processing. The tem¬ 
perature of the material approaches the temperature of the bath 
or retort very quickly. Any variation in the method of packing 
which interferes with convection alters the rate of change of tem¬ 
perature in the center of the can. The proportion of liquid to 
material is important, as has been shown under “ Sweet corn.” The 
fineness of division of the material is of importance because of the 
increased effect of surface tension in finely divided material. The 
fineness of division also affects the proportion of liquid to material. 

The blanching or precooking affects the temperature changes if it 
in any way alters the nature or proportion of free liquid in the ma- 



Fiu. 51. Time-temperature relations for different vegetables when processed in No. 3 
tin cans at 100° C., as compared with distilled water. Curves A, B, and C were plotted 
from readings made at intervals of 1 minute, and curves D to H were plotted from 
readings made at intervals of 5 minutes. A, Distilled water ; B, string beans ; G } soy 
beans ; D, cabbage ; E, tomatoes ; F, pumpkin ; G, corn ; H, sweet potatoes. 

terial. Blanching the leafy vegetables would enable a closer pack 
to be made and would thus make the rate of change of temperature 
slower. 

The heating and cooling medium is of importance. When the 
container is heated or cooled in the air the process is very slow. 
When it is heated in water or steam and when cooled in water, the 
process is very rapid, depending upon the other factors already 
pointed out. 

Figure 51 shows curves for a large number of vegetables proc¬ 
essed in No. 3 tin cans at 100° C. They fall pretty distinctly into 
two groups. The group having a free liquid with a consequent 
rapid rise in temperature contains by far the larger proportion of 
fruits and vegetables. This group includes string beans, Lima 
beans, soy beans, peas, and asparagus. In the second group, having 














































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 45 


very little free liquid, the rate of change of temperature is very slow. 
In this group are sweet potatoes, sweet corn, pumpkins, and summer 
squash. Tomato and cabbage form a somewhat intermediate group. 

INTERMITTENT PROCESSING. 

Since sterilization by the intermittent process depends not only 
upon the maximum temperature attained, but also upon the length 
of the interval between processing periods and upon the temperature 
during this interval, it becomes of very great importance to under¬ 
stand thoroughly the time-temperature relations throughout the en¬ 
tire process. The first processing is supposed to destroy all vegeta¬ 
tive forms of bacteria, and during the following interval any spores 
which may be present germinate and are killed during the second 
processing period. Any spores failing to germinate during the first 
interval are expected to germinate during the second interval and so 
are destroyed in the vegetative form during the third process. If 
the temperature during these intervals should be either too high or 
too low for the germination of any spores, then the whole process 
might fail. It is also known that spores of certain bacteria under 
optimum conditions germinate very quickly, multiply, and again 
form spores in a period of less than 24 hours. These facts make it 
highly important to understand the entire time-temperature rela¬ 
tions. 

In the experiments on the intermittent process a record of the 
processing temperatures, the temperature of the air to which the cans 
were removed after processing, and the temperature at the center of 
the can was kept during the entire period of 72 hours. The length of 
the processing period was exactly 1 hour. This treatment was given 
once on each of three successive days. String beans, corn, soy beans, 
and sweet potatoes were tested in this way. 

STRING BEANS. 

The varietv of bean used was the Green Pod Stringless. The beans 
were washed and broken into pieces 1 to inches in length and 
blanched for five minutes in the boiling-water bath. They were cooled 
and packed into the cans and enough 2 per cent brine added to cover 
the material. They were then processed, as above stated. Figure 52 
shows the results for No. 2 and No. 3 tin cans, and for pint and quart 
glass jars for the entire period. 

During the first processing the temperature of the No. 2 and No. 3 
tin cans approached that of the bath in about 10 minutes, the pint 
jar in about 25 minutes, and the quart jar in 30 minutes. The order 
of their heating up was No. 2 tin cans first, No. 3 tin cans next, then 
pint glass jars and quart glass jars last. The No. 2 and No. 3 tin 


46 


BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 


cans remained at about 100° C. for a period varying between 40 and 
50 minutes each, and the pint and quart glass jars for a period be¬ 
tween 25 and 30 minutes. At the end of one hour they were taken 
immediately from the bath and put in the air. The cooling was sIoay. 
The temperature fell to 60° O., as follows: No. 2 tin can, 1 hour and 25 
minutes; No. 3 tin cans, 1 hour and 40 minutes; pint glass jars, 1 





60 30 /OO /SO /W /60 /BO 200 220 240 260 280 300 320 

T/ME IN MINUTES 



Fig. 52.—Time-temperature relations for string beans in various containers when proc¬ 
essed for 1 hour on each of three successive days (the intermittent process) at 100° C. 
in the boiling-water bath : A, First day ; B, second day; G, third day ; a, No. 2 tin cans; 
6, No. 3 tin cans ; c, pint glass jars ; d, quart glass jars ; x, temperature curve for water 
bath ; y, temperature curve for room. The interval between the end of the curves in 
A and the beginning of the curves in B was 18 hours and 40 minutes. The same period 
of time elapsed between the end of the curves in B and the beginning of the curves 
in C. 

hour; and the quart glass jars 1 hour and 20 minutes. The order of 
their cooling was pint jars fastest, quart jars next, then the No. 2 
tin cans, and No. 3 tin cans slowest. 

Since the optimum temperature for the germination of most bac¬ 
terial spores is between 30° and 40° C., the length of time that the 
can remains at this temperature is important. It was found in this 
































































































































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 47 

case to be between 1 and 1^ hours, which is probably sufficient to 
allow the germination of most spores. The temperature remained 
below 30° for the remainder of the 24 hours. 

During the second processing the time-temperature relations were 
practically the same as during the first. The rate of change of tem¬ 
perature was very slightly slower, but this was insignificant. 

The result of the third processing was essentially the same as the 
second. There was no change in the first heating, which affected 
materially the rate of change of temperature in the second and third 
processing. 

SWEET CORN. 

The variety of sweet corn used in these tests was Stowell’s Ever- 
green. Ears which were in prime condition for canning were selected 
in the field. They were husked, the silks were removed with a coarse 
brush, and they were then washed in water. The corn was prepared 
“ Maine style ” and enough brine-sugar solution (2 per cent salt and 
6 per cent sugar) was added to make the proportion 4.5 of corn to 

1 of liquor. It was then processed for exactly one hour on each of 
three successive days. The time-temperature curves for No. 2 and 
No. 3 tin cans and for pint and quart glass jars are shown in figure 53. 

The results shown here are very different from those for string 
beans. The temperature, instead of rising rapidly, went up very 
slowly. In no case did it reach 100° C. The temperature was rising 
when the cans were removed from the bath and continued to rise 
for a considerable time after being placed in the air. The pint glass 
jar went highest, the No. 2 tin can next, then the quart glass jar, 
with the No. 3 tin can lowest. The order in which they heated up 
was different from that of string beans. All the cans went above 
80° C., which is sufficient to destroy most vegetative forms of bac¬ 
teria. The cooling was quite slow. The temperature fell to G0° C. 
about as follows: No. 2 tin can, 2 hours and 5 minutes; No. 3 tin can, 

2 hours and 20 minutes; pint glass jar, 1 hour and 25 minutes; and 
quart glass jar, 1 hour and 50 minutes. The temperature of the con¬ 
tainers remained between 30° and 40° C., as follows: No. 2 and No. 3 
tin cans, 3 hours; the pint glass jars, 2 hours and 20 minutes; and the 
quart glass jars, 2 hours and 45 minutes. For the remainder of the 
time the temperature remained below 20° C., reaching 19° over night. 

The initial temperature in the second processing was lower than in 
the first, and consequently the maximum temperature reached was 
lower. The curves took nearly the same form the second day as the 
first, showing that there had been but little alteration in the rate of 
change of temperature at the center of the can. The results were 
quite similar to those of the first processing. The results for the third 
processing were essentially the same as for the second. 


48 


BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 


SOY BEANS. 

The variety of soy beans used in these tests was the Easy Cook. 
The beans were closely approaching maturity, as the pods were 
beginning to turn yellow. After gathering from the field they were 
placed upon trays and steamed five minutes at 100° C. This softened 
the pods so that they could be easily shelled by hand. The shelled 





when processed for 1 hour on each of three successive days (the intermittent process) 
at 100° C. in the boiling-water bath: A, First day ; B, second day ; G, third day; 
a. No. 2 tin cans ; b, No. 3 tin cans ; c, pint glass jars ; d, quart glass jars ; x, tempera¬ 
ture curve for water hath ; y, temperature curve for room. The interval between the 
end of the curves in A and the beginning of the curves in B was 18 hours and 40 
minutes. The same period of time elapsed between the end of the curves in B and the 
beginning of the curves in C. 


beans were then placed in the cans and enough 3 per cent brine 
was added to cover the beans. They were then processed for exactly 
1 hour on each of three successive days. Figure 54 shows the time- 
temperature record for the entire period of 72 hours for No. 2 and 
No. 3 tin cans and for pint and quart glass jars. 
































































































































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 49 

During the first processing there was a very rapid rise in tempera¬ 
ture. The No. 2 tin can was fastest, with the No. 3 tin can next, then 
the pint glass jar with the quart glass jar slowest. The temperature 
approached 100° C. promptly, thus subjecting the material to that 
degree of heat for a considerable length of time. The cooling was 
rather slow, the pint jar being fastest, the quart jar next, then the 


At 



/o 


O 20 <40 60 30 /OO /20 /OO /60 /SO 200 220 240 260 230 300 320 

T/ME //V M/NUTES 




T/AfE //V A//AZOTES 


Fig. 54. — Time-temperature relations for soy beans (Easy Cook) in various containers 
when processed for 1 hour on each of three successive days (the intermittent process) 
at 100° C. in the boiling-water bath : A, First day ; B, second day ; C, third day ; 
a, No. 2 tin cans ; b, No. 3 tin cans ; c, pint glass jars ; d, quart glass jars ; x, tempera¬ 
ture curve for water bath ; y, temperature curve for room. The interval between the 
end of the curves in A and the beginning of the curves in B was 18 hours and 40 minutes. 
The same period of time elapsed between the end of the curves in B and the beginning 
of the curves in C. 

No. 2 tin can, with the No. 3 tin can slowest. The temperature fell 
to G0° C. in about 1 hour and 15 minutes in the pint glass jar, in 
hours in the quart glass jar, in 1 hour and 35 minutes in the No. 2 
tin can, and in 2 hours in the No. 3 tin can. The temperature re¬ 
mained between 30° and 40° C. in the different containers as fol¬ 
lows : Pint glass jar, hours; quart glass jar, 1 hour and 45 minutes; 















































































































































50 BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 

No. 2 tin can, 2 hours and 25 minutes; and No. 3 tin can, 5 hours. 
The temperature fell to about 20° C. over night. 

The time-temperature relations were entirely different in the 
processing on the second day. The temperature change was very 
much slower, as during the first processing the soluble proteins 
cooked out into the liquid to such an extent as to form a colloidal 
jelly. The change in consistency of the liquor was such that all con¬ 
vection was prevented in the second processing, and the heat passed 
in only by conduction. The temperature curves for the second heat¬ 
ing were almost exactly the same as those of sweet corn. As in sweet 
corn, the temperature did not reach 100° C. at any time. The third 
processing was entirely similar to the second. 

The very slow rate of change of temperature in the second and 
third periods of processing increased very greatly the possibility of 
an incomplete sterilization. Therefore, a period longer than 1 hour 
for the second and third processing would decrease the possibility of 
spoilage. 

SWEET POTATOES. 

The variety of sweet potato used in these tests was the Nancy Hall. 
The potatoes were washed, steamed for 30 minutes, peeled, and al¬ 
lowed to cool. They were then ground in the food chopper in order 
to get a uniform mash commonly known as “ pie stock.’ 1 This was 
placed in the cans and processed for 1 hour on each of three succes¬ 
sive days, as in the preceding tests. Figure 55 shows the time- 
temperature records for No. 2 and No. 3 tin cans and for pint and 
quart glass jars. 

The rate of rise in temperature was very slow, in no case reaching 
100° C. during the processing. The maximum temperature attained, 
between 80° and 90° C. in the different containers, was reached at a 
considerable time after removal from the processing bath. The high¬ 
est temperature was reached in the pint glass jar, the No. 2 tin can 
being next, then the quart glass jar, and the No. 3 tin can the lowest. 
The cooling was as might be expected from material of this sort. The 
temperature remained above 80° C. long enough to destroy most 
vegetative forms of bacteria. The results of the second and third 
period of processing were entirely similar to the first. 

FACTORS INFLUENCING THE RATE OF CHANGE OF TEMPERATURE. 

All those factors discussed under the heading “ Single-period 
processing ” apply to the first period of the intermittent process. 

In addition to these factors the first period of the processing has 
an effect upon the material as treated during the second and third 
periods. As already observed in soy beans, there is often a change in 
the material during the first processing period which greatly affects 


TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 51 


the temperature changes in the second period. To show clearly the 
differences that sometimes occur the following experiment was car¬ 
ried out. 


Apples, pumpkin, and sweet potatoes were cut into half-inch cubes. 
These were placed raw in No. 3 tin cans and enough water added to 
cover the material. Also, soy beans were packed in cans in the same 
way and enough water was added to cover the material. These were 



T/ME //V MINCITES 




Fig. 55.—Time-temperature relations for sweet potatoes in various containers when 
processed for 1 hour on each of three successive days (the intermittent piocess) at 
100° C. in the boiling-water bath : A, First day ; B, second day ; C, third day ; a, No. 2 
tin cans ; b, No. 3 tin cans ; c, pint glass jars ; d, quart glass jars ; x, temperature curve 
for water bath ; y, temperature curve for room. The interval between the end of the 
curves in A and the beginning of the curves in B was 18 hours and 40 minutes. The 
same period of time elapsed between the end of the curves in B and the beginning of 
the curves in C. 


then processed in the water bath for 1 hour. On the following day 
they were processed a second time. Figures 56 and 57 show the results. 

In the pumpkin very little difference in the time-temperature 
curve for the first and second processing is noted. During the first 
period the rise of temperature was rapid, and it was almost equally 
so in the second processing. 

In the apples there was a very marked slowing down of the rate 
of change of temperature, owing to the cooking out of pectin which 
changed the viscosity of the liquid filling the interspaces, lhe in- 











































































































52 BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 

creased viscosity interfered with convection and thus cut down the 
rate at which heat penetrated to the center of the can. 

In the sweet potatoes the differences noted are extremely great. 
During the first processing the free liquid surrounding the pieces of 

material was converted into 
a starch jelly by the cook¬ 
ing out of the starch dur- 
ing the processing, so that 
during the second process¬ 
ing the heat passed to the 
center only by conduction. 

In the case of the soy 
beans the same change of 
rate of rise in temperature 
is noted, though the ma¬ 
terial that changed the vis¬ 
cosity of the liquid filling 
the interspaces between the 
beans was protein, and not 
pectin or starch. 

As heretofore noted, any 
material which alters the 
viscosity of the liquid fill¬ 
ing the interspaces between the pieces of material will affect the rate 
of change of temperature in the can. It may depend upon the going 
into solution of pectins, starch, proteins, or any other mucilaginous 
material. 

If sweet potatoes or 
pumpkins are ground 
in a food chopper and 
packed closely in the 
cans, there is no dif¬ 
ference in the first 
and second heating. 

If the material at the 
outset is of such a 
nature that all con¬ 
vection is stopped, 
then the going into 
solution of starch or 
protein has little effect 
upon the temperature 
changes in the can. 

Any change in the material which affects the freedom of convec¬ 
tion affects the rate of temperature change. It may be the solution 



Fig. 57. —Diagram showing the. effect of the second proc¬ 
essing of different vegetables upon the rate of change 
of temperature. Curves A and D were plotted from 
readings made at intervals of 5 minutes ; curves B and 
C were plotted from readings made at intervals of 1 
minute. A, Soy beans ; B, pumpkin ; G, apple ; D, sweet 
potatoes. 



Fig. 56.—Diagram showing the effect of the first 
processing of different vegetables upon the rate 
of change of temperature. These curves were 
plotted from readings made at intervals of 1 
minute. A, Soy beans; B, pumpkin ; C, apple ; 
D, sweet potatoes. 

























































TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 53 

of some viscous substance, as in the soy beans; or it may be simply 
a compacting of the material, as happens sometimes in the case of the 
leafv vegetables; or it may be the evaporation of the liquid, as might 
happen sometimes in glass where an absolutely tight closure can not 
be made. 

SUMMARY. 

(1) The mercury thermometer is sufficiently accurate for practical 
work in the determination of temperature changes in the canning of 
food materials if it is properly calibrated and standardized. 

(2) A satisfactory apparatus has been devised for measuring the 
temperature changes at the center of the can during the processing 
period and the subsequent cooling, which permits the use of the mer¬ 
cury thermometer both in the water bath and in the steam retort. 

(3) In a can packed with material having an interspace filled with 
a free liquid, as in string beans, the rate of change of temperature 
at the center of the can is very rapid, and in materials of a heavy 
or pasty nature, as in sweet corn, the rate is very slow unless 
mechanical agitation is employed. 

(4) In canned materials the character of the pack and the compo¬ 
sition of the material A^ery largely determine the rate of change of 
temperature in the can. The fineness of division and compactness 
of the material and the amount and viscosity of the free liquid are 
the factors which influence the rate of change of temperature. Vari¬ 
ations in the composition of the material, however, have very little 
effect if the consistency of the material is such that no convection 
can occur. 

(5) Sodium chlorid has very little direct effect upon the rate of 
change of temperature in the can. Dilute sugar solutions have only 
a small effect, but the concentrated solutions have a considerable 
effect in retarding the rate of change. Solutions of starch have a 
very marked retarding effect upon the rate of change of temperature 
at the center of the can. The retarding effect increases very rap¬ 
idly from 2 to 5 per cent. In 5 per cent starch the consistency 
becomes such that all convection is stopped and the rate of change 
is very slow. Increasing the percentage of starch further has very 
little effect upon the temperature changes. Also, any other material 
of a viscous nature, such as protein or pectin, retards the rate of 
change of temperature. 

(6) The glass container has a marked retarding effect upon the 
rate of rise in temperature in those materials in which there is a 
free liquid, as in string beans, but is of little importance in materials 
of a heavy consistency, such as sweet corn. On the other hand, glass 
cools faster in the air than tin, owing to its greater power of 
radiation. 


54 


BULLET[N 956, U. S. DEPARTMENT OF AGRICULTURE. 

(7) Differences in the diameter of the container are of much less 
importance in those materials in which there is a free liquid than in 
materials of heavy consistency. Thus there need be little difference 
in the processing period of No. 2 and No. 3 tin cans of string beans, 
but there must be considerable difference in the processing period 
of No. 2 and No. 3 tin cans of sweet corn. 

(8) The temperature of the bath or retort is reached in the 
container in approximately the same time, whether the processing 
temperature is 100°, 109°, 116°, or 121° C. Tomatoes are a striking 
exception to this rule, because the higher temperatures break down 
the tissues of the fruit. 

(9) The difference in the rate of cooling in the air and water is 
very marked. In materials having a free liquid the cooling is 
exceedingly rapid, as in string beans, but is considerably slower in 
materials having a heavy consistency, as in sweet potatoes. Cooling 
in air is always very much slower than cooling in water. 

(10) Since a steam-tight closure in glass containers can not be 
made, any temperature above 100° falls to 100° as rapidly as the 
temperature of the retort, so that the temperature is always 100° 
or below when removed from the retort. 

(11) In the intermittent process, the first processing period may 
or may not affect the rate of temperature change in the second 
processing period, depending upon the composition and nature of 
the material. Any change during the first processing period which 
interferes with convection retards the rate of change of temperature 
during the second processing period. This change may be the 
simple compacting of the material, the going into solution of starch, 
protein, pectin, or any other mucilaginous material. If the material 
at the outset is such that no convection occurs, then the gelatinization 
of starch or other such change has very little effect upon the rate 
of change of temperature in the can. 

(12) The fruits and vegetables as processed in these tests fall 
roughly into two groups, with reference to time-temperature rela¬ 
tions. The first group consists of those fruits and vegetables packed 
so that there is a free liquid filling the interspaces between the 
pieces of material. The rate of change of temperature in this 
group is very rapid. The second group consists of those materials 
that are packed in such a way that little or no convection can occur. 
The rate of change of temperature in this group is very slow. 


LITERATURE CITED. 


(1) Belser, Joseph. 

1905. Studien fiber verdorbene Gemiisekoiiserven. In Arcli. Hyg., 
Bd. 54, Heft 2, p. 107-148. 

(2) Bitting, A. W. 

1912. The canning of foods; a description ot the methods followed 
in commercial canning. U. S. Dept. Agr., Bur. Chem. Bui. 
151, 77 p. 

(3) 1915. Methods followed in the commercial canning of foods. U. S. 

Dept. Agr. Bui. 196, 79 p., 3 pi. 

(4) -and Bitting, K. G. 

1917. Bacteriological examination of canned foods. Nat. Canners’ 

Assoc. Bui. 14, 47 p., 22 fig. 

(5) Bovie, W. T., and Bronfenbrenner, J. 

1919. Studies on canning. An apparatus for measuring the rate of 
heat penetration. In Jour. Indus, and Engin. Chem., v. 11, 
no. 6, p. 568-570. 

(6) Castle, Carrie E. 

1919. Effect of pack and depth of water bath upon interior tempera¬ 
ture of jars in cold-pack canning. In Jour. Home Econ., v. 11, 
no. 6, p. 246-251. 

(7) Deming, C. L., ed. 

1902. Science and experiments as applied to canning. 172 p., illus. 
Chicago. 

Contains a number of papers on sour corn by S. C. Prescott and 
W. Lyman Underwood. 

(8) Denton, Minna C. 

1918. What temperature is reached inside the jar during home canning? 

In Jour. Home Econ., v. 10, no. 12, p. 548-552. 

(9) Duckwall, E. W. 

1905. Canning and Preserving of Food Products with Bacteriological 

Technique . . . v. 1, illus., pi. Pittsburgh, Pa. 

(10) Haselhoff, E., and Bredemann, G. 

1906. Untersuchungung fiber Konservenverderber. In Landw. Jahrb., 

Bd. 35, Heft 3, p. 415-444. 

(11) Kochs, J., and Weinhausen, K. 

1908. Welche Temperaturen erreichen Obst- find Gemfisekonserven 
beim Sterilisieren. In Ber. Iv. Gart. Lehranst. Dahlem, 
1906/07, p. 146-161. 

(12) Prescott, S. C., and Underwood, W. L. 

1898. Contributions to our knowledge of microorganisms and steriliz¬ 
ing processes in the canning industries. In Tech. Quart., v. 11, 
no. 1, p. 6-30, 6 pi. 

(13) Thompson, Geo. E. 

1919. Temperature-time relations in canned foods during sterilization. 

In Jour. Indus, and Engin. Chem., v. 11, no. 7, p. 657-664, 
9 fig. 

(14) ZA VALLA, J. P. 

1916. Canning of Fruit and Vegetables • • • 214 p., illus. pi. New 
York. 



ADDITIONAL COPIES 

OF THIS PUBLICATION MAY BE PROCURED FROM 
THE SUPERINTENDENT OF DOCUMENTS 
GOVERNMENT PRINTING OFFICE 
WASPUNGTON, D. C. 

AT 

15 CENTS PER COPY 


V 







LIBRARY OF CONGRESS 



ft 


7 

4 




Jk 











































































